AR Varmint Guns and Buffer spring acting like kinetic hammer

[BobbyJ]

I just put together a 24" bull barrel AR. Right now I have a AR15 lower on it with standard carbine buffer and spring.

I put a few empty 53gr vmax and 69 gr SMKs and let the action slam a round in. I ejected and checked the AOL, I find that it slams them so hard the Bullet actually moves forward a little. OAL changed from 2.250 to 2.255" Does the same on my other ARs.
This is new Star-line brass I took out the ex pander ball to get even more neck tension but does not stop it 100 percent.

So I'm thinking could it be my buffer springs are too strong? Maybe a weak spring so it does not slam the rounds home quite as hard?

My kinetic bullet puller works the same way so I guess its sort of expected movement with a AR. I could gently single feed to avoid the issue.

Maybe it happens to everyone, not many people probably eject live rounds to measure. So maybe its a non issue.

 

[Walt Krafft]

The operating dynamics of an AR can be quite complex with many variables effecting the cycling speed. Including, gas port size and location, barrel length, buffer tube length, buffer spring, buffer weight, bolt carrier weight, powder charge, powder speed, and probably other things. Loading ammo from a magazine instead of single loading can make a big difference. All of these things need to be working together for the gun to cycle properly. As far as the bullet moving, it probably happens more than we know. I have shot thousands of rounds without a crimp and without checking. What is the target telling you? Controlling ammo to some specific jump in a semi-auto fed from a magazine is probably not worth the effort imo. Chop the spring off and see what happens. It might solve your problem, it might create a new one.

 

[Oso]

What you are experiencing is not uncommon. Yes, an AR15 bolt slamming forward is like a kinetic hammer. For this reason the neck tension needs to keep the bullet from shifting forward approximately .003"-.004" range. Another common solution is crimping the cases. A tapered crimp die is handy and you only need a light crimp.

The main concern with the bullets shifting forward is the risk that a bullet may get lodged in the barrel when extracting a loaded cartridge. Aside from the cartridge being loaded into the chamber, the back/forth hammering of the BCG can also cause bullets to shift in the cartridges loaded in the magazine. This can cause feeding issues as a cartridge becomes too long and starts binding/rubbing the front of the magazine.

Just something to test. Looking at cartridges after they are fed into chamber as well as those in the magazine. Keep in mind the forward momentum/force is greatest from the BCG cycling after a fired round, and not just closing the bolt from the locked open position. Shoot the rifle and let it cycle in the next cartridge... extract this unfired cartridge and measure it for how much bullet might have shifted. Repeat. Next, take a few more shots and then check the length of cartridges in the magazine to see if they are shifting after several rounds (5 to 10 shots).

 

[ckaberna]

I just put together a 24" bull barrel AR. Right now I have a AR15 lower on it with standard carbine buffer and spring.

I put a few empty 53gr vmax and 69 gr SMKs and let the action slam a round in. I ejected and checked the AOL, I find that it slams them so hard the Bullet actually moves forward a little. OAL changed from 2.250 to 2.255" Does the same on my other ARs.
This is new Star-line brass I took out the ex pander ball to get even more neck tension but does not stop it 100 percent.

So I'm thinking could it be my buffer springs are too strong? Maybe a weak spring so it does not slam the rounds home quite as hard?

My kinetic bullet puller works the same way so I guess its sort of expected movement with a AR. I could gently single feed to avoid the issue.

Maybe it happens to everyone, not many people probably eject live rounds to measure. So maybe its a non issue.

Look at it this way, you should be at or slightly into the lands every round. IF the projectiles are long enough.

 

[BobbyJ]

Look at it this way, you should be at or slightly into the lands every round. IF the projectiles are long enough.

So you are saying don't worry about it as the lands will stop it if it moves too much.

I was thinking more in accuracy terms. If some bullets are 2.250 and some are 2.260 it might affect the velocity or POI messing up accuracy.

 

[BobbyJ]

What you are experiencing is not uncommon. Yes, an AR15 bolt slamming forward is like a kinetic hammer. For this reason the neck tension needs to keep the bullet from shifting forward approximately .003"-.004" range. Another common solution is crimping the cases. A tapered crimp die is handy and a light crimp is more than adequate.

The main concern with the bullets shifting forward is the risk that a bullet may get lodged in the barrel when extracting a loaded cartridge. Aside from the cartridge being loaded into the chamber, the back/forth hammering of the BCG can also cause bullets to shift in the cartridges loaded in the magazine. This can cause feeding issues as a cartridge becomes too long and starts binding/rubbing the front of the magazine.

Just something to test. Looking at cartridges after they are fed into chamber as well as those in the magazine. Keep in mind the forward momentum/force is greatest from the BCG cycling after a fired round, and not just closing the bolt from the locked open position. Shoot the rifle and let it cycle in the next cartridge... extract this unfired cartridge and measure it for how much bullet might have shifted. Repeat. Next, take a few more shots and then check the length of cartridges in the magazine to see if they are shifting after several rounds (5 to 10 shots).

Thanks, yea I will check it out. I probably will single feed some and compare to slam feeding if the precision is the same I guess its not a big deal. I took my neck expander out to get the max tension and they still move a little, the 69s move more.
The 55gr with crimp ring on the bullets dont move at all. I use standard Dillon 223 dies they do have a taper crimp but it had little if any effect. If you over crimp it will just smash the neck of the case as its applying downward pressure to get the crimp.

 

[jelenko]

As Walt Kraft mentioned, before you spend more effort on keeping the bullet from moving during recoil and/or feeding, what are your targets telling you about accuracy?

From my and many others' experience with AR's with match grade barrels and assembled reasonably well, 1/2 MOA groups is relatively common fired from both mags and single loaded.

FYI, generally, crimping does not increase accuracy potential.

 

[BobbyJ]

As Walt Kraft mentioned, before you spend more effort on keeping the bullet from moving during recoil and/or feeding, what are your targets telling you about accuracy?

From my and many others' experience with AR's with match grade barrels and assembled reasonably well, 1/2 MOA groups is relatively common fired from both mags and single loaded.

FYI, generally, crimping does not increase accuracy potential.

Agree I don't think crimping does much at all unless the bullet has canulare ring. Its just to prevent movement and setback.
If I get 1/2 MOA I will be very pleased.

 

[Minshooter]

If your primary concern is having ammunition that functions 100% of the time in your autoloaders,
the Lee Factory Crimp Die actually works quite well, and is not as case length critical as a roll or taper crimp.
Used with a cannelure bullet, it comes very close to duplicating military ball loadings in push resistance.
Used without a cannelure bullet, Lee Engineering ads used to claim it makes no difference in accuracy.
Some years back there were some "advertising wars" with bullet companies who said it does.
In my tests with "Average" to Match Grade Ar's, and a few bullets available with and without a cannelure,
with and without the Lee FCD, there was no definitive difference on the targets.
Some slightly better, some slightly worse, either way. Not enough to determine a definite difference.
ES and SD was slightly lower with the FCD in both cannelure and no cannelure bullets.
The Crimp (actually a four point staking) was done much lighter on non cannelure bullets.
Tests were with practical supported shooting, typically expecting ~1" at 100yds, ~2" at 200, ~3" at 300, etc.

Bench Rest is another world entirely; nothing above is intended to apply to that.

 

[dellet]

Keep in mind the reason or cause the bullet is moving.

In theory, the operating system is designed to work from -40 to hot enough to cook off rounds. With a chamber full of mud, ice, carbon old oil and half a sandwich stuffed in the magazine. While maintaining a rate of fire above 900 rounds a minute, before you need to pick up a rock and beat on that big button on the side the upper to chamber a round.

This is balanced by providing enough gas to pull the empty case out you just hammered in. If it can’t get the whole case, it will at least pull the back half off.

If you can’t pull the trigger that fast, maintain your rifle a bit better than described above or stay out of firefights in the local swamp, there is a lot you can do to slow down the bolt speed and force that goes into chambering a round from the magazine and maintain reliable feeding.

It all starts at the gas port, location and size. If you regulate the pressure and volume of gas, you can slow down the bolt speed and or lighten the mass And still have reliable feeding in normal shooting conditions. Less force means less bullet movement.

Crimping tighter may stop the bullet from moving, but won’t stop the shoulder from being set back, which also happens quite often.

Less recoil, faster follow up shots. A couple of good reasons to have a well tuned AR.

 

[David Milisock]

I just put together a 24" bull barrel AR. Right now I have a AR15 lower on it with standard carbine buffer and spring.

I put a few empty 53gr vmax and 69 gr SMKs and let the action slam a round in. I ejected and checked the AOL, I find that it slams them so hard the Bullet actually moves forward a little. OAL changed from 2.250 to 2.255" Does the same on my other ARs.
This is new Star-line brass I took out the ex pander ball to get even more neck tension but does not stop it 100 percent.

So I'm thinking could it be my buffer springs are too strong? Maybe a weak spring so it does not slam the rounds home quite as hard?

My kinetic bullet puller works the same way so I guess its sort of expected movement with a AR. I could gently single feed to avoid the issue.

Maybe it happens to everyone, not many people probably eject live rounds to measure. So maybe its a non issue.

Of course the bullets get bounced around in an auto loader and like it or not it affects your jump to the lands. Does it have negative impact? Maybe, I'd put my money on it!

You can crimp, set neck tension or feed single. However you can't change the fact that it's an auto loader.

All the things you can try would fill 4 pages of a post and most likely would be an exercise in futility. Here's my experiance, my Aero Precision AR15 M4E in 6MM ARC, with 10 rounds dropped into a magazine, from a bench at 100 yards prints them into 1/2". This is a $1,300 rifle not a match grade so judge what the effort might gain you on your end.

 

[Pawnee Bill]

Get you a Silent Capture Recoil Spring from JP rifles and you will enjoy your AR a lot more!!

 

[David Milisock]

Get you a Silent Capture Recoil Spring from JP rifles and you will enjoy your AR a lot more!!

The new rifles are much more enjoyable without that TWANG THUD of the old A1 and A2's.

 

[Coyotefurharvester]

I was interested by this thread. I had on hand loads with Nosler 40 Bt, Hornady 50 Sx, Sierra 55BK, Hornady 60 vmax.
Rifle carbine buffer tube/buffer/cheap spring. 18" Wyde chamber barrel. The 55 and 60's for a bolt gun just cleared the 10rd magpul(so 2.25-2.26 coal). Which should be long for this rifle. Chambering off the magpul with at least 5 rds in it. Coal shorten by 0.001-0.0015 slight rifling and ramp markings. Seated some of the rounds 0.025 shorter coal. Chambering off same mag configuration resulted in 0.001-0.002 coal increase. I did notice a difference between chambering off the left vs right side. Right side produced more bullet movement and ramp marks.
When I assembled this rifle I used some dummy rounds(50 gr Sx) and filed/polished the ramps until there weren't any ramp feeding marks. These dummy rd coal were 2.220. Trial chambering some of my 50 gr Sx winter hunting loads saw no change left side feed, 0.0005-0.001 right side of mag. The Nosler 40 Bt no coal change from the 2.233(these also for one of my bolt guns) however slight ramp marks off right side, left side strip/chamber appears no markings.
All range brass I deprimed, washed, sized, trimmed to minimum deburred, mandrel .222. I will look for a Lee taper crimp die and see if it will eliminate bullet movement(none of these bullets have a cannelure) of these slick side bullets. Trend seems bullet weight and starting length related with some magazine stripping position influence, at least with my 0.002 neck interference sizing. I guess I could get a 0.003 and 4 thousandth under pin gage and try more neck tension also.

 

[RegionRat]

Seated some of the rounds 0.025 shorter coal.

Coyotefurharvester, any idea of the seating force you would see if you measured the rounds you seated for these tests?

I see value in using an inline Wilson style seater with an arbor press that has a force pack to help beginners learn how to set "neck tension" and friction controls when learning to reload for the AR platform.

Nowadays, I drag out the AMP Press so they don't have to study a bouncing gage needle as they seat, but the concepts are what is important. There is a direct correlation between the seating forces and these issues, so measuring the force is a way to judge where you stand with your neck prep.

Many Mil-Specs include both crimp and neck sealant, concepts which don't help us with prairie dog ammo. The "friction coefficient" of cured neck sealant runs almost independent of the neck tension in Mil ammo. We do/did run tests on the crimp and seating forces as independent QC checks, but in the end the sealant covers many sins.

You don't "have to have" these things, but they do help illustrate the issue directly. One can always test using their rig, (always remember to play it safe with live ammo).

 

[moorepower]

I never crimp and have never had a round stick, and have never noticed an accuracy issue. Most varmint bullets will shoot from .010-.040 jump with little effect on accuracy.

 

[Coyotefurharvester]

No arbor, no force gage. All brass was washed(inside necks clean no carbon) spray lubed 1 shot. Mandrel I.d. necks 0.002 under bullet diameter. My take, 1. bullets under 50 gr tend to "stay put" 2. While I did get my ramps cleaned up during assembly(with my intended hunting bullets). Longer bullets and longer coal showed some sharp/rough feed ramps. 3. Magazine ammo feed position plays apart(at least with this magazine/upper). This was just a quick mess around check on ammo stability with a rifle whose primarily use is winter hunting. So my brass is clearanced 0.003-0.004 and I use the 10rd magpul magazine, I don't want to have to forward assist rds (or have a unfired rd/bullet stuck in chamber) in below zero weather. Rifle is consistently sub moa in current setup/load off a bipod. Really all I need, considering the barrel was 50.00 new, whole rifle under 350.00.

 

[Oso]

I have not bothered using a crimp on my reloads, which usually include a BTHP type bullet. Using an expander mandrel to set neck tension is all you need for a boat-tail bullet. However, flat-based bullets don't like balancing on case mouths and may require flaring the case mouth to aid in initial seating and eliminate shaving of the copper jacket. Recently, I started playing with using an NOE Plug on some bulk reloads to size the case neck and simultaneously flares the case mouth so that a flat-based bullet just fits inside the case mouth without tipping over. After seating the bullet, I use a very light tapered crimp to iron out any case mouth flaring.

Crimping is a trade-off. You get surety of a secured bullet, but the crimp may introduce some variation to neck tension from cartridge to cartridge. Good enough for hunting, plinking and self defense purposes, but maybe not consistent enough for precision and long range.

 

[BobbyJ]

It all starts at the gas port, location and size. If you regulate the pressure and volume of gas, you can slow down the bolt speed and or lighten the mass And still have reliable feeding in normal shooting conditions. Less force means less bullet movement.

Less recoil, faster follow up shots. A couple of good reasons to have a well tuned AR.

I don't plan on fast shooting a varmint gun. What you said makes sense. You want to go with the lightest BCG that will reliably shut the action. Which then means you might have to adjust the gas port.

For now I just have a standard BCG just added a new bolt so it wears into the new barrels chamber vs swapping bolts from my ARs which would work.

What adjustable low profile block works best for this. I just haev a standard Aero Low profile block on it.

 

[BobbyJ]

I have not bothered using a crimp on my reloads, which usually include a BTHP type bullet. Using an expander mandrel to set neck tension is all you need for a boat-tail bullet. However, flat-based bullets don't like balancing on case mouths and may require flaring the case mouth to aid in initial seating and eliminate shaving of the copper jacket. Recently, I started playing with using an NOE Plug on some bulk reloads to size the case neck and simultaneously flares the case mouth so that a flat-based bullet just fits inside the case mouth without tipping over. After seating the bullet, I use a very light tapered crimp to iron out any case mouth flaring.

Crimping is a trade-off. You get surety of a secured bullet, but the crimp may introduce some variation to neck tension from cartridge to cartridge. Good enough for hunting, plinking and self defense purposes, but maybe not consistent enough for precision and long range.

If you look at a dillon powder drop on .223 it does not flare the case. I think I will just avoid flat based bullets. The only bullets I've reloaded for .223 are the Hornday 55gr FMJ and the 69gr both are boat tail and easy to reload with on the Dillon. I just got 200 of the 53gr they are BT too.