Building a 7RSAUM for 3 different bullets - dumby rounds and Google Sheets

[steffen707]

Long time lurker, first time poster.


I’m building a suppressed 7RSAUM "Jack of all trades, master of none" from a 7RM Tikka T3x and want to shoot 3 bullets out of it. I have an 18” proof .284 barrel ready to be chambered.

Brass:

• I’ll be using 200 pieces of ADG brass (Going to a level 1 course at Vortex Edge and need 150 reliable/consistent cartridges)

Bullets:

• 145gr Barnes LRX
• 168gr Nosler Accubond LR
• 190gr Berger LR Hybrid

I bought a box of each bullet and slit 3 case mouths so I could easily move the bullets in/out.
I made the 3 dumby rounds and then built an excel calculator to see if I could replicate my physical findings with math. Here’s what I came up with:



Background:

• This will be primarily a hunting rifle and want to keep the LRX bullet as fast as possible so that it’ll still have 2000fps at distance to open up as designed. If LRX doesn’t like my rifle, then I’m hoping the ABLR will. I’ve never hunted out west but having a rifle/shooter capable of 500yards on game is the goal. Since the Berger and LRX are so far apart, I assume there could be many bullets inbetween that I could test if LRX and ABLR don't work out.
• I wanted to dabble in long range target to see if I can do it/for fun, and I don’t have money for a dedicated hunting rifle and dedicated long range target rifle right now.
• If the 7RSAUM doesn’t work out to 500 yards for game, I can always rechamber/rebarrel for something bigger and then I’ll know if I like LR target shooting so I don't have to build a target/hunting rifle a second time.

Constraints:

• Want bullet width of bearing surface in the neck (in my case .284”)
• Loaded cartridge needs to fit in the magazine when barrel new, and when barrel has eroded. (I’m not chasing lands Erik Cortina (nice videos by the way, made total sense to me), but enough buffer in case I need to move the bullet out to chase the NODE, or if the bullets I’m using have some variation in length, I don’t want to be right at 3.3” mag length, i want some space.
• Bullet needs to be far enough out that I can seat it deeper if needed to find node.
• Each bullet may have a different seating node “distance to the lands”, I’ll use a guess that should work for all bullets.

Concessions:

• If I made this a dedicated hunting rifle, I could use a cartridge with more powder capacity and still stay in the 3.3” magazine, like my 7LRM, but i'd still have possible issues with the 168ABLR needing to sit too far into the case.
• I'm having trouble finding quality 7LRM brass but have found 7RSAUM ADG or straight .284 Win Lapua.
• I could get a longer action to use all 3 bullets in 7RM, but I already have the Tikka, Mesa stock and pic rail for it.
• 18” barrel is short, might not have the velocity I need/want, but I’m hoping it’ll still be enough and be a light and portable rig. 18” may be loud, but I’ll be running suppressed hunting and at the range. If I fall in love with target shooting and decide to compete (doubtful) in a class that requires no muzzle device, I’ll build another rifle before I would compete.

Questions:

• Is expecting .030” of available bullet movement out of the case to “chase the NODE” too little, too much?
• The gun will be a Tikka with 7mm bullet, the rest i'm open to suggestions.
• Any glaring issues you see other than “18” Barrel is SHORT”, or “I hate LRX bullets”.

 

[steffen707]

This is my excel calculation. I basically built the 3 dumby rounds, measured 3 sections of each bullet, picked a desired starting distance from ogive to the lands/jam and then calculated the rest. It was suprisinly close to the dumby rounds. Like within a few thousandths and that could easily have been error when I was calculating bullet base to wedge (not sure what you call it, but where the bearing surface of the bullet starts).

 

[Rook 6]

Looks like you are putting a lot of effort into your research and I am sure folks will reply with a lot more in-depth knowledge and experience than I. In my mind, there are not a lot of absolute answers to your questions. Is .30 enough to chase the node? Probably. Too much? Not sure there is such thing as too much. Just depends on your rifle. For sure you want to keep an eye your OAL vs mag length like you said. If the round is too long, even fitting in your magazine, the recoil can really do a number on the point of your bullets. Any particular reason you picked the three bullets you chose? You mentioned using them for hunting and PRS. Based on that, the Berger would be a likely candidate for both, but might end up being uncomfortably long still. Have you considered the Berger 180? It might be a better candidate to meet your application criteria?

 

[Lance21]

As far as hunting and PRS the Berger 190 does both, although the recoil will be substantial for PRS. I run the 190s @ 2885, 24 inch barrel. Stiller Predator xm sized action.

 

[steffen707]

Looks like you are putting a lot of effort into your research and I am sure folks will reply with a lot more in-depth knowledge and experience than I. In my mind, there are not a lot of absolute answers to your questions. Is .30 enough to chase the node? Probably. Too much? Not sure there is such thing as too much. Just depends on your rifle. For sure you want to keep an eye your OAL vs mag length like you said. If the round is too long, even fitting in your magazine, the recoil can really do a number on the point of your bullets. Any particular reason you picked the three bullets you chose? You mentioned using them for hunting and PRS. Based on that, the Berger would be a likely candidate for both, but might end up being uncomfortably long still. Have you considered the Berger 180? It might be a better candidate to meet your application criteria?

I've read you don't want to seat the bullet any deeper than the base of the neck (where a donut can form) and also takes away powder capacity. With a 2.301 Base to Ogive on the 190gr Berger (with my comparator, your mileage may vary), i'll still have the ability to seat the bullet out further by 0.270 (to chase the NODE) and still have a full 0.284" of bullet bearing surface in the neck. At this Base to ogive, the COAL is only 3.1 which is 0.200" shorter than mag length.

My calculated Base to ogive of each bullet is within 0.056" of each other, so none of them should be intruding on the powder capacity, unless I find one of the bullets likes a lot of jump.

I picked the 145gr Barnes LRX because I subscribe to the camp of retained bullet weight, knocking a hole through the animal for tracking if needed, quartering to/away shots. I also don't like the idea of a bullet fracturing into a million pieces of lead that my family and I might injest. (though i've shot those bullets for years and haven't died yet).

I picked the 168 ABLR because it has a pretty high BC and if the Barnes doesn't work out, at least its bonded and retains more weight than other bullets.

I picked the 190gr Berger because it has a higher BC than the 184 Berger. I also have 2 boxes of the 184 Bergers and since they're lighter, if I can make the 190Berger and 145LRX work from the same gun, pretty much anything inbetween can work too.

 

[steffen707]

As far as hunting and PRS the Berger 190 does both, although the recoil will be substantial for PRS. I run the 190s @ 2885, 24 inch barrel. Stiller Predator xm sized action.

I'm not going to compete in PRS or Fclass with this rifle, just wanted to know what its like to shoot far and wanted a high BC bullet to try it out on.

Can you run a muzzle break or suppressor in PRS? Do you use one and the rifle still has a lot of recoil? What cartridge are using?

 

[Clancy]

You did say that you were dead set on the tikka and 7mm, but open to suggestions. Here it goes.

Shoot the 190 Berger exclusively. Berger hybrids are one of the very best hunting bullets made, and they won’t trash your barrel with copper like that Barnes will. I don’t have anything against the Nosler, except that it’s hard to find.

If that barrel is carbon, ditch it. 18” is short, but carbon barrels and magnums means you’re going to be getting a shot in about every 15 minutes, and target shooting is pretty much out of the question due to recurring accuracy issues and thermal mirage. Ask me how I know. If you’re dead set on carbon, get a Bartlein. Reports are better across the board, although I think I’m done with carbon barrels for good except 22 RF.

Seat your dummy with the bottom of the bearing surface at the neck shoulder junction, and have the chamber cut so that’s 30-50 thousandths off the lands. You’ll have lots of room to chase.

Fun project, and I hope you know I’m speaking from experience when I say build the gun around one bullet and avoid those carbon proofs. They look cool and some of them shoot well, but they are far far from worth the trouble.

 

[steffen707]

You did say that you were dead set on the tikka and 7mm, but open to suggestions. Here it goes.

Shoot the 190 Berger exclusively. Berger hybrids are one of the very best hunting bullets made, and they won’t trash your barrel with copper like that Barnes will. I don’t have anything against the Nosler, except that it’s hard to find.

If that barrel is carbon, ditch it. 18” is short, but carbon barrels and magnums means you’re going to be getting a shot in about every 15 minutes, and target shooting is pretty much out of the question due to recurring accuracy issues and thermal mirage. Ask me how I know. If you’re dead set on carbon, get a Bartlein. Reports are better across the board, although I think I’m done with carbon barrels for good except 22 RF.

Seat your dummy with the bottom of the bearing surface at the neck shoulder junction, and have the chamber cut so that’s 30-50 thousandths off the lands. You’ll have lots of room to chase.

Fun project, and I hope you know I’m speaking from experience when I say build the gun around one bullet and avoid those carbon proofs. They look cool and some of them shoot well, but they are far far from worth the trouble.

Thanks Clancy! This gives me something to ponder.

 

[Dub]

Agree a Carbon wrapped barrel will be hard to pin down on repeated firings.

 

[JB10]

I too want to build this cartridge but have decided to wait till components are a little easier to find. Took me a good portion of this year just to get stuff for my 6.5 prc.

 

[Clancy]

I too want to build this cartridge but have decided to wait till components are a little easier to find. Took me a good portion of this year just to get stuff for my 6.5 prc.

I’m due another barrel and had to go 6x47 again instead of BRA, because Varget and 4895 are so hard to find. I guess I’m lucky in some ways, I stumble on H4350 rather often.

 

[Coyotefurharvester]

What is the twist of your barrel, 1/9" or faster? Zero to 1/4 of SAAMI freebore will help keeping OAL around 2.8". 2700 fps maybe obtainable with 190's.

 

[steffen707]

I’m due another barrel and had to go 6x47 again instead of BRA, because Varget and 4895 are so hard to find. I guess I’m lucky in some ways, I stumble on H4350 rather often.

I hear H4350, H4831sc, H1000 are some to try in the 7RSAUM

 

[steffen707]

What is the twist of your barrel, 1/9" or faster? Zero to 1/4 of SAAMI freebore will help keeping OAL around 2.8". 2700 fps maybe obtainable with 190's.

1:8" twist.

 

[steffen707]

I too want to build this cartridge but have decided to wait till components are a little easier to find. Took me a good portion of this year just to get stuff for my 6.5 prc.

Ever use, or heard a buddy use Whidden Dies? I think i'm going to take a chance on their set if I go 7RSAUM. @Clancy has me questioning if I should just build this tikka as a 7RM for the 145LRX and call it good.

 

[Clancy]

I hear H4350, H4831sc, H1000 are some to try in the 7RSAUM

H1000 and SAUMS go hand in hand, but the last time I saw an 8 lb jug of that a Bigfoot and a Leprechaun were fighting over it.

I’m not gonna press any harder on your bullet choice. I ultimately needs to make YOU happy, but I’d sure rather shoot the hybrids than that nasty Barnes.

I think 7 SAUM is a great choice, you’ve already got a 7 LRM, don’t need the belted 7 mag if you don’t just “want one,” which is reason enough!

 

[Lance21]

I'm not going to compete in PRS or Fclass with this rifle, just wanted to know what its like to shoot far and wanted a high BC bullet to try it out on.

Can you run a muzzle break or suppressor in PRS? Do you use one and the rifle still has a lot of recoil? What cartridge are using?

Yes you can run either one. I'm running the 190s out of a 7 saum. I use a ec tuner brake on it, does a good job considering the rifle weighs 9 lbs.

 

[mac86951]

Hmm, I'm hearing two "flags" that are worthy of a few questions.

1. short action and 190s? The SAUM really responds well to longer seated heavy bullets, and i would think the 190 would benefit; though it's doable from a 2.825" mag; it's similar to a 77gr in a 223; takes up a lot of boiler room. I'm meaning that 2700fps from a 24" barrel would be what I'd expect is very near max and you're not going to want to see what the brass looks like with an 18" barrel and 2700fps.

2. 18" barrel? 7saum and 7RM (30saum and 30WM) are near equivalent; and I'm sure that the 18" barrel will suffer reduced max velocities with h1000; even h4350 may not burn well leaving a large fireball for the suppressor. It's absolutely doable; but the combinations of components are countering what I'm thinking your ultimate goals are. Yes the bullets will make it to 1000y, even at far slower velocities no concerns there.

I'd recommend 24" min barrel length, it can always be cut shorter; but my barrel stretcher design is still in the works; right next to the board stretcher and wire lengthener. In my experience, SAUM velocities are really hindered by shorter barrels. Again, it's doable; but with a short barrel, that's taking away some of the magnum advantage and maybe a different cartridge would be a better idea.

-Mac

 

[Clancy]

I should add that I assumed you’d be getting the longer bolt stop and running longer mags for the SAUM. 190s in a true short box doesn’t sound like fun.

 

[JB10]

Ever use, or heard a buddy use Whidden Dies? I think i'm going to take a chance on their set if I go 7RSAUM. @Clancy has me questioning if I should just build this tikka as a 7RM for the 145LRX and call it good.

Iv just been useing Redding dies for all my stuff. I am not a benchrest shooter and dont want to spend crazy money on all typed of dies. Most benchrest guys use willson dies anyways.