(COAL/CBTO) How Short is TOO Short?

[TribbleRED]

Funny question, right?

The common knowledge approach would have a guy or gal seat that bullet as far forward as they can - usually well past saami - especially if you're single loading or you have a magazine that will accommodate the length - which I have neither option. After a fairly extensive search I couldn't seem to find the info for which I am looking so I am turning to you fine folks to help me out with a conundrum and perhaps get some specific information as it pertains to my case(no pun intended)...

I need to fit a 7saum in an AR10 Magpul mag and that roughly puts the COAL ~about~ 0.1" shorter than saami. I say "roughly/about" because while it could be longer than the 0.1" and still "fit" in the Magpul mag the tight tolerances could end up causing feed issues in the magazine if the magazine, say, was not in pristine condition all the time. On that note if you have a recommendation for another AR10 mag that could get me longer length in the AR10 lower please let me know...

With having to seat the bullet a little deeper into the case: what am I looking at as far as issues with pressure or chambering?

Additionally, I'll likely have the folks at McGowan Precision Barrels build the barrel and they will want 3 dummy rounds as reference for the chambering so I won't be dealing with working around a saami chamber. At the moment I seem to be only limited by the magazine - so what fits in there will have to fit in the chamber but if its going to cause a problem with pressure(which is really my question here) then I am going to have to tackle this another way.

7saum saami COAL: 2.825
Magpul COAL(what makes me comfortable for reliable feeding): 2.7460

I am hoping to use Berger 168 VLDs(or heavier), Hornady 166 A-Tip Match, 175 ELD-X, or 180 ELD Match and still be at 2.7460 and 3,000fps out of a 28" barrel with a retumbo or similar slower burning powder... tall order, I know, but at the end of the day I'll shoot whatever bullet weight I need to to get those velocities. I just figured those answering would like some info on what sort of bullet I would prefer so they can get an idea for how much of it is going to be nesteled up inside the case.

 

[nn8734]

See if you can find the M118LR Magpul AR10/SR25 mags. They allow for a bit more OAL vs the standard ones.

Here’s a link

MAGPUL PMAG GEN M3 M118LR 25LR .308 / 7.62x51 Magazine

Optimized for use with 7.62x51 NATO M118LR or other .308 Winchester match ammunition with a maximum over-all length of 2.830" Product #: MP MAG577-BLK

magcommander.com

 

[918v]

If the ogjive falls below the case mouth then your round is too short. Knights Armament mags allow up to 2.855” COAL. They are the best AR10 mags out there.

 

[nn8734]

Agree, I only use KAC metal mags; have had occasional feeding issues w/AR10 PMags

 

[TribbleRED]

Good recommendation on the mags. I sincerely appreciate it. I had the KAC and the Larue mags on my list to eventually move in that direction but I figured the pmags were the safest bet to start given my excellent experience with them (ver. m3/gen3). If this build works well the next barrel would consider longer COAL allowed by longer mags. My ONLY apprehension is I have no knowledge on how longer mags will reliably feed these rounds - that's really it.

Does anyone know of any adverse pressure caused by seating a bullet a little deeper..? Is it actually problematic or is it really just getting in the way of adding more powder...? At the end of this inquiry I will determine the length of this cartridge which will determine the length of throat, hence importance beyond just my pressure related question.

 

[nn8734]

Just do a proper load work up looking for pressure along the way. deeper seating will raise pressures assuming identical charge weight, primer, sizing etc. but there’s no reason you can’t develop a safe, reliable load.

 

[TribbleRED]

Just do a proper load work up looking for pressure along the way. deeper seating will raise pressures assuming identical charge weight, primer, sizing etc. but there’s no reason you can’t develop a safe, reliable load.

I figured as much but I wanted to be certain that I wasn't missing anything here...

 

[TribbleRED]

You're shooting 7 SAUM in an AR10? That's pretty ambitious. How are you getting around the bolt face? What will your barrel length be and what do you think your MV will be?

Luckily I wont have to build my own bolt as KAK makes a bolt for the .450 Marlin which shares the same .535 bolt face that is required by the 7 saum. I am looking at a 28" barrel to shoot the 168 VLD with a goal of 3k fps MV - That's the plan anyways. As the rounds will be limited by magazine length my seating depth will have to be taken into account when load developing. At the end of the day I want to find the best accuracy with the heaviest bullet I can muster making that 3k-3.1k fps... We'll see what happens. I do expect the Mk1 to be promising but not the final product.

 

[spife7980]

Realize that the 450 marlin is a 43k peak pressure round.
To get a 7 saum to 3k is going to be 65k psi.

Id call KAK and see if they feel their system can support an addition 50% in pressure on the top end.

 

[TribbleRED]

Realize that the 450 marlin is a 43k peak pressure round.
To get a 7 saum to 3k is going to be 65k psi.

Id call KAK and see if they feel their system can support an addition 50% in pressure on the top end.

Good catch my man. I will get ahold of them and find out.

 

[TribbleRED]

From KAK:

it will handle 65. it's the same bolt we use for wsm


kurt


Kurt Kosman

President

KAKindustry.com

Office: 805.981.4734



Thanks for asking the question, Spife.