Velocity Deviation Problems

[pcso112]

Having some problems with my velocity deviation with 220 and 240 SMK’s. After doing some reading I understand that this is not a unique problem for the larger subsonics. My horizontal groupings are sub-moa, but my vertical groups are as large as 2.5 moa and I am seeing 80+ fps deviation in my velocities. Painstaking care has been taken to insure that all my cartridges are perfectly uniform.

I am using a 21-inch Hart barrel with 1 in 8 twist on a Remington 700 action. My suppressor is a YHM 762 Phantom. I have been loading up cut down 5.56 06 LC brass. I use CCI BR2 primers and have had the same velocity deviations with both H110 and Reloader 7.

It also appears that the 240gr SMK’s are not stabilizing and aren’t flying straight. It is apparent on my targets that about half are landing off center.

I am headed to town this weekend and will be picking up a different powder. My question is, can anyone tell me is there a particular powder that might be better suited toward my 21 inch barrel for subsonics?

[amafrank]

Sounds like a different powder is in order. Barrel lengths make a great deal of difference in what powder does what. I can't use the loads from the 16.25" remington in the 9" SIG and expect the same results. I know its obvious that velocities will be lower in the shorter barrel but consistancy is different too. It seems 2400 worked fantastic in the short barrel with average deviations of 9fps using the 220's. The same powder in the longer barrel was great at 25-30fps but the lil'gun seemed to be the one in that length for minimum speed spread. Still not sure which is the most accurate since I can't keep a scope working long enough to get good data...need to quit buying cheap scopes....
I'm guessing that with the way long barrel you have you are going to have to do some testing with more than a few powders to get it right. 21" is great if you want max velocity but for subs I'd think shorter is better.

Frank

[too_many_hobbies]

I was having trouble with the first shot in a 5 shot group being 40 to 70 fps slower than the next 4 shots. The first shot would impact the target 1.5" to 4" lower than the next 4 shots. The last 4 would group within an inch. For the last 4 shots I was getting a very small extreme spread sometimes as small as 5 fps. This phenomena would repeat itself anytime I waited at least an hour between firing groups.

It turned out that setting the shoulder back slightly by adjusting my full length resizing die down about one half turn solved the problem. The first shot being slower and impacting lower problem went away but my extreme spread went up to about 20 fps.

All the above was with 220 Sierras and Vihtavuori N110 at subsonic velocities.

[pug]

One possible fix for the first round being way low is case annealing. I can't say it is the fix and the annealing kit would set you back about $45. You don't really need the kit but it is a good way to get started. Another method is to get a Lee .223 lock stud and spin it with a cordless drill or screwdriver. Lock the case in the holder, turn on the drill and put the map gas torch to it until the neck is orangey colored. If it goes red it is too hot and will fail early. Lower pressure subsonic rounds made from toughened mil brass may not be sealing. The first shot may not seal the chamber completely but with the aid of heat the subsequent shots are able to completely seal the chamber. By pushing the shoulder back further (giving extra headspace) none of the shots fully seal the chamber and therefore "smooth out" the readings but the velocity spread is higher. Not saying its the problem just one more variable to look at.