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300 subsonic probl's - Throwing the 1st rnd low every time

Discussion in 'Rifle Discussion' started by Remadl700, Aug 18, 2013.

  1. Remadl700

    Remadl700 Oregon Member

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    I am pulling my hair out. I have a Noveske 300/221, 16.5 barrel, 1-8 twist and a AAC Cyclon supressor. While out at the range I ran some super sonic 165 Noler's through the gun with no problem. Then switched over to the HSM 220 sub sonic 300 rounds and fired 3. THe first one went 4 inches low and the last 2 were touching at the POI. Tried it again, loaded the 20 Bushmaster mag with 3 rounds and fired the group. Almost exactly the same outcome, 1 went 4 inches low and the other 2 at POI and touching....?????? so I switched over to my Sub sonic rounds. 220 HPBT Sierra with 13.5 of H335, COL at 2.26. Again the first round went about 5.5 and fired 5 more that were touching at POI.....????......the one thing that was noticeable was that my loads hit the paper unstable due to the impact print. As you would expect I went through the gun and mag's. Inspected the barrel, suppressor and mag. Everything looked fine. Suppressor was tight, no visible damage, the bores were clear nothing rattling. Disassembled the gun and re-assembled it. Ran 3 more HSM rnd's through it and the same damn thing.???? so changed mags, NO change. I am at a loss........the only thing that i can determine is that the 1st round is loaded into the chamber by charging the bolt and the rest of the rounds are loaded due to gas pressure....????? shouldnt make a difference but I am at a loss.

    Please anybody.....................I have been loaded for several years but I am by know means an expert. If you want I can supply pics of the target that i shot.
     
  2. hoody

    hoody Tigard/Beaverton area Active Member

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    Just thinking aloud here. Shoot a group, put on safe, let cool, but don't unload. Shoot next group. Is 1st shot of second group low? If so, that suggests manual vs gas powered chambering isn't the problem.

    Next, quickly shoot two groups, both loading 1st round via charging handle. 1st round of second group low = suggests not a cold bore thing.

    If not those things, check the mirror?
     
  3. Twodogs

    Twodogs portland Or Active Member

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    Single feed 3rds and see if they group.
     
  4. Remadl700

    Remadl700 Oregon Member

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    to scary...lol

    sounds like a plan. Another thing that is frustrating is that the HSM 220 subsonic rounds are punching paper perfectly at 1025-1045 fps off the chrono but my rounds that are 220 Sierra HPBT at the same speed with H335 are accurate but are showing unstability based on the way they punch the paper. With the 1 in 8 twist i don't know why this is.


    let know the outcome..
     
  5. AMProducts

    AMProducts Maple Valley, WA Jerk, Ammo Manufacturer Silver Supporter

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    technically a 220gr bullet needs about a 1:4 twist to be truly stable at those velocities.

    In all honesty, I think your H335 load is probably a bit "slow" for that round/velocity combo, and you might be better off with something like h322, or switching over to a faster burning IMR type powder. (4198 performs very well in 7.62x39, but it might not have enough port pressure to cycle the bolt, this is the trade-off when it comes to running "subsonic")

    If you want to know if you have a good load or not, chrono about 20 rounds, if your standard deviation is a single digit number, you're doing good. If it's larger than 15 your load needs work and isn't performing properly.

    Also, I don't expect most guns to deliver more than about 1.5-2 MOA without custom loads, lots of tuning, and then you need to fire heater/fouler shots before each group in order to ensure a consistent barrel condition. a 5" diversion from POI seems a bit extreme, but as you said this could be caused by your charging. Typically I "charge" the gun, and use the BHO to lock it open, insert magazine, let the bolt fly, and keep running it that way.
     
  6. Dan1477

    Dan1477 Beaverton, Oregon Active Member

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    Take the suppressor off and see if you get the same results.
     
  7. H2O MAN

    H2O MAN Georgia - USA Well-Known Member

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    Try warming the barrel with two supersonic rounds first.
     
  8. Remadl700

    Remadl700 Oregon Member

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    John built this for me back in 2006 and set it up for Subsonic rounds. Its a pistol length gas tube and enlarged gas port. The load I am using is one that he and Joel originally gave me along with several others. I have a suspicion that I know what the problem is now but have to do some work first. I will let you all know the outcome.

    Thank you all for your sugestions.