6.8 SPC and Twist Rate

I cross posted this at 68 Forums and I’m adding it here for posterity.

Here’s the rub: Twist rate makes ZERO difference to barrel performance.  My earlier theoretical analysis appears to be supported and I expect that twist rates equal to or slower than 1:4 will see no appreciable difference in pressure or muzzle velocity. So far I haven’t seen any difference with 16″ 4-groove barrels in 1:7 vs 1:11 twist.

The numbers (mv in average of several shots):

16″ 6.8 BSP”

SSA 115 OTM: 2492 fps

SSA 110 TSX: 2547 fps

SSA 85 TSX TL: 3010 fps

16″ 6.8 SPC II 1:11″

SSA 115 OTM: 2493 fps

SSA 110 TSX: 2575 fps

I had exactly 5 rounds of the 85 TSX TL and didn’t have any left for the SPC II 1:11″ twist. I’ll get those next time. I didn’t measure accuracy closely but I was shooting about 1.25″ to 1.5″ at 105 yards with everything. Neither barrel showed better accuracy than the other. The brass looked exactly the same. With a suppressor or without, didn’t make any difference.

More testing will follow with the SSA Tactical loads

 

18″ SPR 7.62mm / .308 Winchester Range Report

We put our new 18″ SPR 7.26mm/.308 Winchester rifle through its paces at the range this week. This rifle features our .308 Winchester compatible 7.62mm NATO chambered barrel. The overall performance was flawless. The Chrony was set up 15 feet from the muzzle, and we loaded up with Hornady 155 gr A-Max and Winchester 168 gr Ballistic Silvertip ammo. The A-Max achieved a muzzle velocity of 2685 fps, and the Silvertips hit 2365 fps. Reliability was perfect. At the end of the session we had a nice pile of .308 brass sitting at 3:30 about 10 feet away. Our MA-TEN based BR-X rifles were designed to go bang every time the trigger is pulled, because we know your hunting trip or your life may depend on it.

The first session showed good accuracy with 1.5 MOA 5-shot groups. Took the rifle out for a second range session because we knew it could do better. The first trip out we found that the trigger was terrible. It was a basic GI single stage trigger, but something was obviously wrong with the mating surfaces between the hammer and the sear. This time out we put a Timney single stage trigger in the rifle with a 4.5 lb pull. What a difference. Here’s our setup:

18" BR-X on the bench

 

And the view down range:

BR-X on the bench, downrange view

Targets are 105 yards from the muzzle. No Chrony on the second day as it was raining and miserable. This time we brought several different types of ammo including 150 gr Hornady SST Superformance, and hand loaded 150 gr SMK and 150 gr Barnes TSX

The Bank

And the results

Range results

Starting at top left: the first group was fired with 150 gr Sierra Match King over 46 grains of H4895. The first shot is for sight in, then moved a few clicks left and called it good for testing. As you can see, there are many three shot groups that would be great, but three shot groups are for sissies, so I manned up and shot 5 in each group. This is more a testament to my shooting still than to the rifle. Our professional shooter Dave wasn’t available for this range day so I had to do the dirty work myself. First group came in at 1.36″, measuring the groups as the largest distance between the centers of any two shots in the group. This produced a respectable 1.34 MOA.

The second group at the top right is 150 gr Barnes TSX over 46 grains H4895. Woof! I didn’t even bother measuring that one. From the left at the bottom: 150 SMK over 43 gr Varget, 150 Hornady SST Superformance, 150 Barnes TSX over 43 gr Varget. The rifle obviously likes the SMK’s, and with some time put into load development, should be able to push MOA or better. This will be our focus in the future. I’m also looking forward to the weather warming up and a nice sunny day at the range. Once I get a good load dialed in, it will be time to call Dave to put up some real groups.

Bison Carbine with Ops Inc 12th Model Silencer

Silencers really make shooting fun. We brought a lightweight 16″ carbine to the range for some fun with an Ops-Inc 12th model silencer.

BR-15 carbine, Ops-Inc 12th Model silencer

Silencers are expensive, the wait for the paperwork to go through is a drag, but they’re worth the trouble. Once you shoot suppressed, you’ll never go back. And you’ll never move to California.

Another new BRT Comp!

This one has the scallops – pictured alongside our other new comp with the smooth side.

 

Expedited Shipping

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Note About Order Status Terminology

This has been a point of confusion, both for us at Bison Armory and for our customers. Here’s how we use the available order status terms:

1. Awaiting Fulfillment: This is the default order status when you place a new order. This status indicates that we haven’t done anything yet with the order.

2. Pending: This order status indicates that we have entered your order in the system and are either waiting for parts that are not in stock, waiting for FFL confirmation for lowers and rifles, or are getting your order together and building any assemblies included in your order.

3. Refunded: A status of “Refunded” indicates is the same as pending except that we have applied any partial refunds that you have coming (e.g. for free shipping, LEO/Military discount, or order changes and errors).

4. Shipped: Touchdown! Your order is on it’s way to you. Your UPS/USPS tracking number is available on your account/order page and you should be receiving emails from both Bison Armory and UPS Quantum view with your tracking number.

Just Arrived! Redesigned BRT Comp

Check out the newly refined styling of our BRT Compensator.

NEW! 16″ RECON 6.8 Gas Piston Rifle

Bison 16" Recon with Mega Monolithic forearm and Adams GPU

This rifle is now available in the Bison Armory store. CLICK HERE to go directly to the product page.

Features: 16″ 6.8 SPC barrel (stainless, bead blast or black Cerakote), Mega Monolithic upper receiver with single or quad rail, Adams gas piston unit, Magpul MOE+ grip and CTR stock, extra smooth trigger group (upgradeable to Geissele SSA or S3G), choice of STAG, LWRC or LMT bolt and Young Manufacturing bot carrier, standard charging handle (upgradeable to Vltor BCM Gunfighter or PRI Gas Buster)

FREE SHIPPING anywhere in the USA (including AK and HI) and your choice of either one FREE PRI mag or 2 FREE YHM mags!!

 

Bison Ops is Live!

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