Sierra 125 SMK in .300 BLK pressure

[MrK.]

Has anyone used the 125 SMK for .300 BLK. I am getting weird pressure spikes using them. I know start low, work up yada yada all combinations of components are different. I picked up some 125 SMK pulls from American Reloading along with some primed brass which turned out to be 95% Ammo Inc. I pushed out the primers to look at them and they appear to be CCI 400s although I know visual comparison is sketchy at best. My go to load with 125 grain bullets in this gun is 17.9 gn of Win 296 with a WSR primer and I typically get 4-5 loads before primer pockets get a little loose. This load is sub MOA using: Nosler BT, Hornady SST, Speer TNT and Sierra SPT Pro-Hunter bullets. I have also used reformed: LC, Winchester, RP, PMC and Hornady brass as well as RP, GFL and PNW Arms .300 BLK brass, it's happy with any of them. This is a middle of the road load according to everyone's data. When I first got the bullets in I checked them for weight, OAL, diameter and all measurements were very consistent and were as expected. My first plan was to do a mini ladder shooting them into the ground outside my workshop. I loaded 17.3, 17.6 and 17.9 gn and all pieces of brass appeared normal. I loaded up 3 at 17.9 and went to the range and two of the three had brass flow into the ejector. I reduced to three each of 17.6, 17.3 and 17.0 and still got high pressure signs. I pushed out three of the primers and inserted some WSRs with the same result at 17.0. I used the three pushed out primers and used some reformed LC 5.56 brass at 17.0 gn with the same result. 17.0 gn is BELOW Sierra's starting charge for 125 SMKs for H110 which I think everyone knows is the same as W296. I attached a pic of the first group I shot at 17.9, second pic is the three pieces of brass with the WSR primers at 17.0. First I blamed the brass (too soft?), then the primers but now I think it's the bullet..... Thoughts?

 

[Steve Donlon]

I'm using Starline brass with 18.3 gr. of 296 and Remington 7 1/2 prime getting 1" groups at 300 yards if everything goes right. That bullet is the most accurate bullet I have shot in my DDM4 Hunter rifle. I'm loading at the book load of 2.210 OAL. They show no pressure signs. Hope you figure it out, that's my favorite load in my AR.

 

[jcm800]

My guess would be the loaded ammo (neck dia) is too large for chamber. Check a piece of fired brass vs your loaded rounds.I have to turn necks when using oversize cast bullets for that reason.

 

[padom]

I've tested 125 smk and even better the 125 TMK.

I use H110 out of a 10.5" Rainier Ultramatch, CCI 450s and converted LC brass. Stupid accurate.


Here 125smk over 18gr H110 when I was breaking in my new Rainier Ultamatch. No load workup. Started top left and shot left to right, dropping down line by line.

https://imgur.com/Hqe471o

And here is my 125 TMK H110 load development

https://imgur.com/GiMGMbg

And here is my 125 TMK 7x5 load confirmation. It straight hammers.

https://imgur.com/x6X3GDa

I then took this 125 TMK load to distance and went 10 for 10 on steel at 200, 300, 400 and 500 meters with my Leupold Mark 4 MR/T 1.5-6 CM-R2... above load development was done with a Razor HD 5-20 FFP

 

[padom]

I'm using Starline brass with 18.3 gr. of 296 and Remington 7 1/2 prime getting 1" groups at 300 yards if everything goes right. That bullet is the most accurate bullet I have shot in my DDM4 Hunter rifle. I'm loading at the book load of 2.210 OAL. They show no pressure signs. Hope you figure it out, that's my favorite load in my AR.

Try the 125 TMK. Most accurate 300blk super load I've tested.. also, Black Hills uses the 125 TMK in their defensive ammo.

My buddy and I did some back yard expansion testing with the 125 SMK and 125 TMK loads I posted above. We shot into denim and wet phonebooks...

The 125 SMK just bent slightly and looked like a banana when recovered. Zero expansion.

The 125 TMK opened up with sharp pedals like a 9mm HST..

Everything was tested out of my 10.5" Rainier Ultramatch (shilen) SBR

 

[MrK.]

My guess would be the loaded ammo (neck dia) is too large for chamber. Check a piece of fired brass vs your loaded rounds.I have to turn necks when using oversize cast bullets for that reason.

Thank you for this suggestion. I checked some neck diameters.

Fired case: .335 (one of the three from my pic above).
Ammo Inc Case with 125 Gn SMK: .330
Converted RP .223 Rem case with 130 Gn Speer HP: .332 in. These are accurate and run fine in this gun save the occasional one that fails to feed because of the big hollow point.

 

[Steve Donlon]

Has anyone used the 125 SMK for .300 BLK. I am getting weird pressure spikes using them. I know start low, work up yada yada all combinations of components are different. I picked up some 125 SMK pulls from American Reloading along with some primed brass which turned out to be 95% Ammo Inc. I pushed out the primers to look at them and they appear to be CCI 400s although I know visual comparison is sketchy at best. My go to load with 125 grain bullets in this gun is 17.9 gn of Win 296 with a WSR primer and I typically get 4-5 loads before primer pockets get a little loose. This load is sub MOA using: Nosler BT, Hornady SST, Speer TNT and Sierra SPT Pro-Hunter bullets. I have also used reformed: LC, Winchester, RP, PMC and Hornady brass as well as RP, GFL and PNW Arms .300 BLK brass, it's happy with any of them. This is a middle of the road load according to everyone's data. When I first got the bullets in I checked them for weight, OAL, diameter and all measurements were very consistent and were as expected. My first plan was to do a mini ladder shooting them into the ground outside my workshop. I loaded 17.3, 17.6 and 17.9 gn and all pieces of brass appeared normal. I loaded up 3 at 17.9 and went to the range and two of the three had brass flow into the ejector. I reduced to three each of 17.6, 17.3 and 17.0 and still got high pressure signs. I pushed out three of the primers and inserted some WSRs with the same result at 17.0. I used the three pushed out primers and used some reformed LC 5.56 brass at 17.0 gn with the same result. 17.0 gn is BELOW Sierra's starting charge for 125 SMKs for H110 which I think everyone knows is the same as W296. I attached a pic of the first group I shot at 17.9, second pic is the three pieces of brass with the WSR primers at 17.0. First I blamed the brass (too soft?), then the primers but now I think it's the bullet..... Thoughts?

It has to be the brass. It's not worth the effort. From your numbers, the brass is to thin.

 

[MrK.]

It has to be the brass. It's not worth the effort. From your numbers, the brass is to thin.

That was my initial thought until I had the same issue with reformed LC .556 brass. To give some more information about the brass. I grabbed 5 of the Ammo Inc brass and 5 pieces of RP .308 BLK brass. The RP brass averaged 84.8 gn and held an average of 25.2 gn of water. The Ammo Inc brass averaged 86.6 gn and held 25.3 gn of water. I also sectioned some just to get a look at the thickness of the web. Picture below is Ammo Inc (L), RP (C), PNW ARMS (R).

 

[Steve Donlon]

That was my initial thought until I had the same issue with reformed LC .556 brass. To give some more information about the brass. I grabbed 5 of the Ammo Inc brass and 5 pieces of RP .308 BLK brass. The RP brass averaged 84.8 gn and held an average of 25.2 gn of water. The Ammo Inc brass averaged 86.6 gn and held 25.3 gn of water. I also sectioned some just to get a look at the thickness of the web. Picture below is Ammo Inc (L), RP (C), PNW ARMS (R).

Could be a bad batch of brass. I had some Stsrline 300 BLK brass the was splitting. Set it back and they replaced all of it. Just a thought.

 

[dellet]

Without complete load data, it’s hard to say. Brass trim length, base to shoulder datum, cartridge overall length would all be helpful.

You are shooting what should be some pretty mild loads, yet only getting 4-5 reloads before the primer pockets give out. That’s just way too hot. I work this cartridge hard and expect 15 firings.

Looking at your fired brass, the brass flow into the ejector hole, and lack of a severely flat primer, points to brass. tho you said it did the same thing with converted LC kind of contradicts that.

Since there isn’t an extractor mark or swipe, I assume this ain’t an AR, but knowing what your shooting this in would be helpful. As would barrel length and velocity.

The 125 SMK loaded to 2.255” will put the base of the bullet right at the bottom of the neck, provided your not resizing to minimum base to shoulder, maximum headspace. At that length using 296, you should be able to safely load to 20 +/- grains and 2250-2300 fps in a 16” barrel.

Back to your statement of only getting 4-5 firings at less than 18 grains using other bullets. Either the bullets are seated very deep, jamming the lands, or you might have a shirt chamber.

I would be checking the length multiple bullets jam.

 

[MrK.]

Without complete load data, it’s hard to say. Brass trim length, base to shoulder datum, cartridge overall length would all be helpful.

You are shooting what should be some pretty mild loads, yet only getting 4-5 reloads before the primer pockets give out. That’s just way too hot. I work this cartridge hard and expect 15 firings.

Looking at your fired brass, the brass flow into the ejector hole, and lack of a severely flat primer, points to brass. tho you said it did the same thing with converted LC kind of contradicts that.

Since there isn’t an extractor mark or swipe, I assume this ain’t an AR, but knowing what your shooting this in would be helpful. As would barrel length and velocity.

The 125 SMK loaded to 2.255” will put the base of the bullet right at the bottom of the neck, provided your not resizing to minimum base to shoulder, maximum headspace. At that length using 296, you should be able to safely load to 20 +/- grains and 2250-2300 fps in a 16” barrel.

Back to your statement of only getting 4-5 firings at less than 18 grains using other bullets. Either the bullets are seated very deep, jamming the lands, or you might have a shirt chamber.

I would be checking the length multiple bullets jam.

Thank you so much for these questions. I will get answers in the next day or two.

 

[MrK.]

Without complete load data, it’s hard to say. Brass trim length, base to shoulder datum, cartridge overall length would all be helpful.

You are shooting what should be some pretty mild loads, yet only getting 4-5 reloads before the primer pockets give out. That’s just way too hot. I work this cartridge hard and expect 15 firings.

Looking at your fired brass, the brass flow into the ejector hole, and lack of a severely flat primer, points to brass. tho you said it did the same thing with converted LC kind of contradicts that.

Since there isn’t an extractor mark or swipe, I assume this ain’t an AR, but knowing what your shooting this in would be helpful. As would barrel length and velocity.

The 125 SMK loaded to 2.255” will put the base of the bullet right at the bottom of the neck, provided your not resizing to minimum base to shoulder, maximum headspace. At that length using 296, you should be able to safely load to 20 +/- grains and 2250-2300 fps in a 16” barrel.

Back to your statement of only getting 4-5 firings at less than 18 grains using other bullets. Either the bullets are seated very deep, jamming the lands, or you might have a shirt chamber.

I would be checking the length multiple bullets jam.

Sorry so slow to answer some of these questions. This is a 16 inch AR15. Part of the reason it's hard on brass is that I shoot a lot of 130 gn Speers and the big hollow point occasionally catches on the feed ramp and jams the bullet. I have alleviated that problem by jamming a piece of #6 lead shot into the hollow point. Weird I know but it has stopped that issue.
I have added some photos to answer the rest of your questions regarding trim length, base to datum and OAL but also measured bullet bearing surface, diameter, length of loaded cartridges and neck diameter of loaded cartridges including a factory round and neck diameter of a fired round. All of these rounds drop easily into and out of my Lyman ammo checker. I also included jam length for the SMK in my chamber. When I resize cases I bump the shoulder about .002" but remember this issue started with new unfired Ammo Inc brass but has occurred with reformed LC and PMC brass as well. The only thing that stands out to me is that the SMK has the longest bearing surface and seem to be slightly oversized. of the 5 I randomly measured they were all .3085-.3090. Maybe that is the issue... these are pulled bullets after all. All other bullets were .308 or less. The last pic shows PMC reformed brass after shooting the 125 SMK at 17gn of 296. I find it very weird that the primers show no signs of pressure yet the brass does... could it be an issue with my bolt?

 

[dellet]

Sorry so slow to answer some of these questions. This is a 16 inch AR15. Part of the reason it's hard on brass is that I shoot a lot of 130 gn Speers and the big hollow point occasionally catches on the feed ramp and jams the bullet. I have alleviated that problem by jamming a piece of #6 lead shot into the hollow point. Weird I know but it has stopped that issue.
I have added some photos to answer the rest of your questions regarding trim length, base to datum and OAL but also measured bullet bearing surface, diameter, length of loaded cartridges and neck diameter of loaded cartridges including a factory round and neck diameter of a fired round. All of these rounds drop easily into and out of my Lyman ammo checker. I also included jam length for the SMK in my chamber. When I resize cases I bump the shoulder about .002" but remember this issue started with new unfired Ammo Inc brass but has occurred with reformed LC and PMC brass as well. The only thing that stands out to me is that the SMK has the longest bearing surface and seem to be slightly oversized. of the 5 I randomly measured they were all .3085-.3090. Maybe that is the issue... these are pulled bullets after all. All other bullets were .308 or less. The last pic shows PMC reformed brass after shooting the 125 SMK at 17gn of 296. I find it very weird that the primers show no signs of pressure yet the brass does... could it be an issue with my bolt?

View attachment 1597012
View attachment 1597013
View attachment 1597014

The numbers look fine, maybe a bit long in the chamber, certainly not short.

The rest is contradicting.

The ammo inc brass shows flow into the ejector. Combined with loose pockets in 4-5 firings seems like high pressure. Like just be garbage brass, but since it happens on LC, it’s questionable.


Other option is someone got happy with a swaging tool to remove primer crimps.
The PMC brass reall does not show pressure. I don’t see where any brass has been pressed into a void on the bolt face, ejector hole. What there is showing are where the brass has smeared. That’s simply over gassed. The case is still stuck in the chamber when the bolt rotates on extraction. Easier for the bolt head to spin on the case head than twist the case in half.

Over gassed, does not mean over pressure. To get the action to cycle with subs, the gas port is automatically too large for supers. No such thing as a 300 Blackout that will cycle a sub load, that is not over gassed for supers.

It’s possible part of the primer pocket problem is also due to over gassing. The case blows out to chamber size, then contracts. If the case is removed before it cools, it won’t contract. With a case head poorly supported already on an AR, it compounds the problem.

This is not real common on a Carbine gas system in a 16” barrel. If it’s a pistol length gas 16” barrel, either an adjustable gas block or trash can are about the only solutions for the barrel. Using pistol gas in a 16” barrel is extremely tough on brass.

The big hollow point having a feeding issue depends on where it hits. Bottom of the ramp out of the magazine, or top where the barrel and extension join. A pic of that might show something. Manufacturers cheaping out on the extension and using 223 ramps instead of 308 ramps is not uncommon.

Any neck diameter .334” and under should not be an issue at all.

Hope that helps some.

 

[MrK.]

The numbers look fine, maybe a bit long in the chamber, certainly not short.

The rest is contradicting.

The ammo inc brass shows flow into the ejector. Combined with loose pockets in 4-5 firings seems like high pressure. Like just be garbage brass, but since it happens on LC, it’s questionable.


Other option is someone got happy with a swaging tool to remove primer crimps.
The PMC brass reall does not show pressure. I don’t see where any brass has been pressed into a void on the bolt face, ejector hole. What there is showing are where the brass has smeared. That’s simply over gassed. The case is still stuck in the chamber when the bolt rotates on extraction. Easier for the bolt head to spin on the case head than twist the case in half.

Over gassed, does not mean over pressure. To get the action to cycle with subs, the gas port is automatically too large for supers. No such thing as a 300 Blackout that will cycle a sub load, that is not over gassed for supers.

It’s possible part of the primer pocket problem is also due to over gassing. The case blows out to chamber size, then contracts. If the case is removed before it cools, it won’t contract. With a case head poorly supported already on an AR, it compounds the problem.

This is not real common on a Carbine gas system in a 16” barrel. If it’s a pistol length gas 16” barrel, either an adjustable gas block or trash can are about the only solutions for the barrel. Using pistol gas in a 16” barrel is extremely tough on brass.

The big hollow point having a feeding issue depends on where it hits. Bottom of the ramp out of the magazine, or top where the barrel and extension join. A pic of that might show something. Manufacturers cheaping out on the extension and using 223 ramps instead of 308 ramps is not uncommon.

Any neck diameter .334” and under should not be an issue at all.

Hope that helps some.

The Speer hollow point caught on the ramp where the barrel and receiver meet. There was a noticeable bump there. I took the gun apart and filed the feed ramp on the barrel extension so the would line up better. Before the file job about one in 5 would jam, after about one in 30. Like I said, the piece of #6 shot has alleviated that issue but I have decided to stop shooting that bullet when I shoot up the last 70 or so I have loaded. That's why I picked up the SMKs. I shot the Speers plinking and the occasional hog and wanted a cheap replacement. I save the Nosler's and Sierra Pro Hunters for shooting deer.

I think you are right about it being overgassed. This is a CMMG wasp upper. I will order an adjustable gas block, give that a go and post results. I put one on my Grendel because it was chewing brass up.
Thanks so much for this!

Edit to add: The Ammo Inc brass has not been reloaded. Its all brand new. I decided last spring that I was going to stop searching in tall grass for .300 BLK brass. It's so cheap to make and I kind of enjoy it actually. To be honest I probably have enough loaded to last me till I die. Just can't stop tinkering.

 

[padom]

The Speer hollow point caught on the ramp where the barrel and receiver meet. There was a noticeable bump there. I took the gun apart and filed the feed ramp on the barrel extension so the would line up better. Before the file job about one in 5 would jam, after about one in 30. Like I said, the piece of #6 shot has alleviated that issue but I have decided to stop shooting that bullet when I shoot up the last 70 or so I have loaded. That's why I picked up the SMKs. I shot the Speers plinking and the occasional hog and wanted a cheap replacement. I save the Nosler's and Sierra Pro Hunters for shooting deer.

I think you are right about it being overgassed. This is a CMMG wasp upper. I will order an adjustable gas block, give that a go and post results. I put one on my Grendel because it was chewing brass up.
Thanks so much for this!

Edit to add: The Ammo Inc brass has not been reloaded. Its all brand new. I decided last spring that I was going to stop searching in tall grass for .300 BLK brass. It's so cheap to make and I kind of enjoy it actually. To be honest I probably have enough loaded to last me till I die. Just can't stop tinkering.
View attachment 1597053

Even easier, order a Rubber City Adjustable BCG for $130 and adjust until it holds bolt open on last round by loading 1 rd in mag at a time....


Or spend $50 and buy an adjustable gas key for your bcg if your mechanicly included to remove your almost always staked gas key screws and installing the RVA gas key. But for $80 if get the complete bcg ..

I used every adjustable gas block under the sun for the past 15yr. I don't use them anymore, I use adjustable gas key Rubber City bcgs and couldn't be happier. So fast and easy to tune..

FYI this is a "cosmetic" blem. But I've bought no less than 35+ of these and NONE arrived in anything other than a normal retail box and no cosmetic blem. Make sure you specify in the notes you want the adjustable gas key installed when checking out as per the directions.

*BLEM* 5.56 x 45(223) Standard Mass-M16 BCG complete - Welcome to Rubber City Armory

***IMPORTANT NOTE*** - PLEASE READ BEFORE SENDING AN EMAIL ASKING WHAT IS ALREADY ANSWERED BELOW: THESE ARE COSMETIC BLEMISHED OR PRODUCT OVERRUNS OF OUR STANDARD MASS 5.56 BCG COMPLETE. - THESE MAY OR MAY NOT HAVE SPECIAL LASER ENGRAVING INDICATING A BLEM STATUS. - DO NOT SEND US EMAILS...

rubbercityarmory.com

Here the standard alone adjustable gas key if you want to go that route.

RCA Adjustable Gas Key - Welcome to Rubber City Armory

Made from Pre-Hardened 4140: 28-32RC with radiused corners reducing sharp edges. Enhances performance of low mass and standard mass GI systems without unnecessary dissassembly. Gas flow is adjusted and maintained at the carrier with set screws. One to adjust gas flow and one to lock it in place...

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[MrK.]

I figured this one out. I tried an adjustable gas block with no luck so I turned my attention to the bullets. As I said earlier they measured .309 so I ran them through a .308 bullet die. This provided no results either, still showing pressure. After I shot that group which printed a couple inches left of center I decided to shoot a group with one of my regular loads which is a 130 Speer HP with 18gn of 296. The first shot blew a primer.... now this has always been a pretty mild load in the past. I got home and ran a scope into the chamber and there was a shiny black carbon coating which I am assuming was reducing overall chamber size causing the pressure. A couple long soaks with free all and a 44cal brass brush cleaned it out. Not really sure how the carbon got all the way down the case body area of the chamber. Anyway I was able to get up to my 17.9gns of 296 which is accurate with every 125gn bullet I have shot in this gun. This is the result after the chamber cleaning.

 

[dellet]

A couple things come to mind. The oversized bullets might be the problem. The transition from the end of the case mouth to the throat has a fairly steep angle. T his spot has caused a lot of problems with cast bullets, it shaves just a little bit of the bullet off as it exits the case. Because of the long jump for many bullets in the cartridge, it was not uncommon early on to end up with a copper ring forming there. Eventually forming a copper crush washer sealing the end of the case. This could also explain the excessive carbon in the chamber.

c heck carefully with your scope.

 

[MrK.]

A couple things come to mind. The oversized bullets might be the problem. The transition from the end of the case mouth to the throat has a fairly steep angle. T his spot has caused a lot of problems with cast bullets, it shaves just a little bit of the bullet off as it exits the case. Because of the long jump for many bullets in the cartridge, it was not uncommon early on to end up with a copper ring forming there. Eventually forming a copper crush washer sealing the end of the case. This could also explain the excessive carbon in the chamber.

c heck carefully with your scope.

View attachment 1605043

Dellet, thanks for your responses to this issue. I have noticed one of those copper rings you mentioned. It was not jammed against the throat as you described, just loose in the action. I'm not sure if it was in this gun but it probably was as this is the gun that gets shot the most. It's kind of my American Express card... Never leave home without it. I purchased a dedicated Dewey chamber rod and some chamber brushes and will try and stay ahead of this carbon problem. Thanks again.

 

[MrK.]

OK... I'm throwing the towel in on this project. I loaded up 5 each of 17.6gn and 17.9gn 296 with the 125 SMK. I went out to the ranch to see which one shot best. Started with the 17.6 load. First 3 shots went into about .3 inch but the third shot left a very light swipe on the brass that I didn't notice until later. 4th shot was about 50fps slower than the rest, shot about an inch above the rest and had some pretty significant brass flow into the ejector. Being the eternal optimist that I am I decided to press on with the fifth shot. The velocity was back up with the rest and the bullet struck a little lower than the fourth but... it blew the primer. This were all single loaded and weren't left cooking in the hot chamber before the shot. I noticed with the other attempts it seemed like the later shots in a group seemed to show pressure. Anyway... like I said I'm done with it. I ordered a new barrel, it will be here Monday. I just hope it groups as well as this one.

 

[padom]

I tested a bunch of 300blk 10-10.5" barrels and had shitty results with all of them and was about to throw in the towel until i said I'll test one more.

Rainier Ultramatch 10.5" 300blk shoots 0.5moa groups all day long with 125smk, 125TMk, 110 Vmax, with h110 and 220smk subs with cfe223....