I have a ar 15 sw mp with upgrade heavy 20 inch 1 8 twist shielen barrel..

The problem im haveing is in three rounds the third round has lost about 75 feet per second..

I started with varget and 55 vmax and went to benchmark and 60 grain seirra hp. both drop about 75 feet per second. about 25 to 50 feet per second per shot.

this is with different loads, differnt bullets.

the shiellen will shoot very well but is very picky about exact case weight and powder amount etc.. 1 /2 inch groups are common, 3/8 and 1/4 inch have been shot, but a little different powder amount or different case weight the group opens up considerably..

I cant seem to be able to go out and repeat what ive shot as a good group, but another load will be as good better..

h335 has been more forgiving but id prefer an extreem extruded powder so im not chasing temp all year long.. ive been shooting in about 20-30 degrees..

I may have to use heavy bullets and h335.

the best group has been with 69 sierra and i think benchmark powder. I could not repeat this group with an almost identical load.. i havent cronied it yet..

Im wondering about gas leaks etc.. Locals say its a powder type problem.. any help will be appreceiated... dave

A 3 shot test group tells nothing and confuses.......

What is the ES of a 10 shot test string (SAAMI standard)? Most ballisticians will tell you the absolute minimum number of shots twhen testing for group/velocity/psi is 7 shots for any chance of reasonable assurance. A 10 shot test string is much, much better and if you are shooting High Power then a 22 shot test string is best.

Larry Gibson

3 shots just shows the bbl heating up.
you need to see if it will settle down during a longer shot string.
if you keep on having velocity loss over a 10 shot string.
and more than one string.
i'd say you have a bbl issue there.
cryo treat might just fix it.