Black Hills 77 grain TMK out of SBR

[Calvin118]

After reading about the enhanced ballistic coefficient and fragmentation range associated with the 77grain TMK's over the traditional SMK's, I decided to pick up a few boxes of the Black Hills load and try it out.

It worked well in my 18" precision rifle and grouped 0.75-1" at 100 yards. As an index, SMK based reloads produce groups of 5/8-7/8"

When I tried it in a 12.5" barrel, however, the best I could do is 3 to 4 inch groups at 100 yards. This upper has a Centurion Arms barrel that shoots the 77grain SMK at just over 1" regularly, and that started the day with a 7/8" 6 shot warm up group with 7-cent Hornady FMJ reloads. After the trouble started I switched back to 77 grain SMK load and the 12.5" barrel again printed a group just over an inch. I then went back to the Black Hills TMK and the trouble returned.

I know the ammo is not bad because the TMK performed well in the 18" barrel.
I know the 12.5" barrel is not bad because it performed well with the Hornady 55gr and 77gr SMK bullets.
I know that it is not me because all the other bullet/barrel combinations performed well with me behind the trigger before and after this happened.

I also know that the 77gr TMK is extremely long; significantly longer than a SMK. In the past, there have been reports of poor accuracy out of SBR's with extremely long bullets such as 70gr TSX due to an inability of the short barrel to stabilize the projectile.

I am beginning to wonder if this is the issue that I am having. I never got very good accuracy out of soft point bullets such as the TBBC or Nosler 64 grain, and terminal performance range out of SBR's is limited in highly accurate bullets such as the 77grain OTM loads. The 77 grain TMK seemed to promise excellent accuracy with extended terminal range, making it an ideal choice for SBR's. Now that is in doubt.

Has anyone else had a similar experience? Is this an idiosyncrasy of my barrel or a design limitation of the TMK?

[DocGKR]

There is a reason why faster twist barrels are needed for some loads: https://pistol-forum.com/showthread....Over-Stabilize

[Unobtanium]

Wolf steel case shoots better than 77gr TMK in my experience. Granted, that's just a couple of 10-shot 100 yard groups, but the stuff sucked. Well over 2 MOA from my DDM4. 77gr SMK from IMI would do 1.5-1.8MOA. Wolf hovers at around 1.8. I have found Gold Dot in all weights to be very accurate, too.

[HCM]

Wolf steel case shoots better than 77gr TMK in my experience. Granted, that's just a couple of 10-shot 100 yard groups, but the stuff sucked. Well over 2 MOA from my DDM4. 77gr SMK from IMI would do 1.5-1.8MOA. Wolf hovers at around 1.8. I have found Gold Dot in all weights to be very accurate, too.

Were you using the 55 grain or 75 grain Wolf?

[Unobtanium]

Were you using the 55 grain or 75 grain Wolf?

55gr Black Box Polyformance

[TiroFijo]

After reading about the enhanced ballistic coefficient and fragmentation range associated with the 77grain TMK's over the traditional SMK's, I decided to pick up a few boxes of the Black Hills load and try it out.

It worked well in my 18" precision rifle and grouped 0.75-1" at 100 yards. As an index, SMK based reloads produce groups of 5/8-7/8"

When I tried it in a 12.5" barrel, however, the best I could do is 3 to 4 inch groups at 100 yards. This upper has a Centurion Arms barrel that shoots the 77grain SMK at just over 1" regularly, and that started the day with a 7/8" 6 shot warm up group with 7-cent Hornady FMJ reloads. After the trouble started I switched back to 77 grain SMK load and the 12.5" barrel again printed a group just over an inch. I then went back to the Black Hills TMK and the trouble returned.

I know the ammo is not bad because the TMK performed well in the 18" barrel.
I know the 12.5" barrel is not bad because it performed well with the Hornady 55gr and 77gr SMK bullets.
I know that it is not me because all the other bullet/barrel combinations performed well with me behind the trigger before and after this happened.

I also know that the 77gr TMK is extremely long; significantly longer than a SMK. In the past, there have been reports of poor accuracy out of SBR's with extremely long bullets such as 70gr TSX due to an inability of the short barrel to stabilize the projectile.

I am beginning to wonder if this is the issue that I am having. I never got very good accuracy out of soft point bullets such as the TBBC or Nosler 64 grain, and terminal performance range out of SBR's is limited in highly accurate bullets such as the 77grain OTM loads. The 77 grain TMK seemed to promise excellent accuracy with extended terminal range, making it an ideal choice for SBR's. Now that is in doubt.

Has anyone else had a similar experience? Is this an idiosyncrasy of my barrel or a design limitation of the TMK?

What is the twist of the 18" upper, and the 12.5"?

[Calvin118]

The 12.5" was a Centurion CHF 12.5" 1/7 twist and the 18" was a CLE chambered Douglas blank with 1/7 twist.

[TiroFijo]

The twist is not likely the reason for the accuracy discrepancy.