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Posted: 12/15/2012 7:39:34 AM EDT
| I just purchased my first suppressor and have a question. I run a 10.5 SBR set up with 1:7 twist noveske barrel. Will 55 Grain bullets shoot just fine through that short of a barrel where I do not have to worry about baffle strikes? Thanks. |
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This is a complex subject, and I am not an expert.
At issue is the ability of the rifling to stabilize the projectile. There are a number of factors involved, including twist rate and linear velocity (with calculated rotational velocity) as well as bullet bearing length. The lighter bullets (40-55 gr) generally have a greater linear velocity than the heavier ones for a given barrel length. One of the experts at Lothar Walther told me: 1:12 twist is for 40-55 gr projectiles 1:9 twist is OK for 55-69 gr projectiles 1:7 twist is for projectiles heavier than 69 gr, including the 77 gr. Because he is an employee of one of the better precision barrel makers, I have no reason to doubt his advice. If the rotational velocity gets too high, there is a possibility of jacket separation, which will definitely seriously destabilize the bullet and cause baffle strikes. This is seen more in the really frangible 55 gr varmint-type loads. I have personal experience with this phenomenon shooting 55 gr Hornady SX projectiles in a 1:9 barrel. My experience is with carbine or rifle length barrels. I have found that the 1:12 in my Winchester 70 gives pretty crappy groups with 62 gr M855 but excellent groups with 55 gr commercial ammo. I get tight groups with an M4 using a 1:7 barrel and 77 gr Black Hills Match, but the groups more than quadruple in size with 62 gr M855. When using a suboptimal bullet/twist combo, there will be some instability and increased yaw depending on the mismatch. Adding a suppressor, whether you will have enough instability with a given bullet/twist combo to get a baffle graze will also depend on the aperture (clearance) within the suppressor. Some manufacturers gain sound performance by tightening the aperture, giving a lot less leeway for slight yaw. Others that were actually designed for military usage have a more generous aperture that will permit minor yaw. I can state that we have seen more issues with the 1:7 10.5 inch barrels than with the 14.5 inch 1:9 barrels when using M855 ammunition. I guess that each owner needs to think through what degree of risk he is willing to take. Personally, if I were using 55 gr ammunition, I would most probably look for a 1:12 (preferably) or 1:9 barrel. The barrels are pretty inexpensive compared to a suppressor. Your call, but if you do have baffle strikes using 55 gr ammo in a 1:7, this may not be a warranty issue. |
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