308 Subsonics

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  • BLS27

    Active Member
    May 1, 2015
    122
    Mt. Airy
    I did some load testing the other day with my 308 subsonics. It's a 24" 1:12 twist. I'm using 180g Hornady SP interlocks, Trailboss, lapua brass and CCI 200 primers. The flash holes are not drilled out, nor are the bullets lubed.

    I'm experiencing a first round drop (2-3" low) with all loads tested. My chrono showed that these were around 100-140fps slower than the last four rounds tested. There was no key hole groups, just drop in velocity. I would shoot the first group, then wait around 7-8 minutes and shoot the next (5 rounds in total). I'm guessing the after the first round the barrel warmed, which is telling me I might need to use lube on the bullets. Even the rounds that were supersonic in the 1200FPS range were having a first round drop issue.

    The group I settled on is 1017 fps average and is plenty accurate and again, no key holes. I'm concern about throwing the suppressor on with the first round shot dropping that low. Is a baffle strike possible with that first round shot or will it just continue to be low? Perhaps it's time to dip these babies in crisco.

    Thoughts?
     
    Last edited:

    DaemonAssassin

    Why should we Free BSD?
    Jun 14, 2012
    23,970
    Political refugee in WV
    It is highly likely that a baffle strike will occur. It is only a function of WHEN the strike happens.

    I really can't see the appeal in wanting to load a subsonic 308. You will get inconsistent ignition of the powder, with that much internal volume of the case not being filled with powder. Load supers, put the can on, and be happy. Also with supers, you will have the peace of mind that a baffle strike is going to be highly unlikely.
     

    normbal

    Ultimate Member
    BANNED!!!
    May 2, 2011
    1,189
    socialist occupied maryland
    I did some load testing the other day with my 308 subsonics. It's a 24" 1:12 twist. I'm using 180g Hornady SP interlocks, Trailboss, lapua brass and CCI 200 primers. The flash holes are not drilled out, nor are the bullets lubed.

    I'm experiencing a first round drop (2-3" low) with all loads tested. My chrono showed that these were around 100-140fps slower than the last four rounds tested. There was no key hole groups, just drop in velocity. I would shoot the first group, then wait around 7-8 minutes and shoot the next (5 rounds in total). I'm guessing the after the first round the barrel warmed, which is telling me I might need to use lube on the bullets. Even the rounds that were supersonic in the 1200FPS range were having a first round drop issue.

    The group I settled on is 1017 fps average and is plenty accurate and again, no key holes. I'm concern about throwing the suppressor on with the first round shot dropping that low. Is a baffle strike possible with that first round shot or will it just continue to be low? Perhaps it's time to dip these babies in crisco.

    Thoughts?

    I've made and shot thousands of subsonics since first getting a .308 suppressor about 20 years ago. There's a LOT of data online about this, particularly from Finland (IIRC) going back many decades.

    My best loads so far are repeatable with an SD of about 12 using lubed 180 grain RN Sierras and about 8 grains of bullseye or 11.8 grains (or thereabouts, YMMV) of Trailboss.

    No, there is NO likelihood of flashover and catastrophic kaboom with these loads. Load one at a time so you don't double charge and even at that, the volume of Bullseye won't do more than get you about 1600 fps (I've done it), and doubling trailboss is really something the mfr already recommends giving data for reduced velocity loads on line.

    Throwing an accurate slug out to 100-200 yds with close to the muzzle energy of a 9mm 115grain round has its benefits, especially when there's almost NO perceptible, localizing muzzle blast from such a round, and with close to 1" of MOA accuracy, taking out deer and varmints with headshots gets boring.

    Good friend of mine has this thing about shooting subsonic .22LR rounds out several hundred rounds from a suppressed 10/22 to take out ground hogs. Works well and accurately.

    There are practical applications of subsonic rounds; several specialty manufacturers sell them (at close to $2.00 a round). You can make them for just a few cents over the price of the bullets alone.

    Baffle strikes are possible, but highly unlikely. Find an online ballistics checker and do the math for 6" of suppressor length; the drop is negligible compared with bore diameter. What you're dealing with is cold bore internal ballistics, not a lot you can do except make the first round a little hotter.

    After you've bought your tenth (or 19th) suppressor, having one dedicated to subsonics makes the potential for loss that much lower compared to the collection.

    Hope this helps.

    Norm
     

    Jmorrismetal

    Active Member
    Sep 27, 2014
    468
    I shoot cast and coated subs for backyard plinkers 11 grains of trailboss is a good place to start for them but I haven't loaded jacketed subs in .308.
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    At what range are you getting the drop of 2-3"?

    If at 100 yards, there is little chance of a baffle strike.

    If at 5 yards, it may.
     

    BLS27

    Active Member
    May 1, 2015
    122
    Mt. Airy
    it was dropping 2" at 100 yards. I did some more reading and found that it's common with a cold barrel to have the first round drop. I can combat this with pushing an oiled patch down the bore first, but F that. I can just run one of my standard loads through it first. I tested again 5 rounds before throwing on the suppressor. The drop was only around 1.5", the rest group around 1 inch. I threw on the can and proceeded to shoot 5 more. This grouped around 3/4" of an inch. I was pretty pleased. Hollywood quiet! I let the barrel cool completely and shot 5 more. The first round only dropped around 1/2" down this time. I'm going to push it to 200 yards next time on steel.

    Thanks for the advice/help.
     

    deerassassin22

    Active Member
    Apr 12, 2016
    700
    Littlestown, PA
    The First String was only 5rnds I was unable to get setup in time. I also was unable to reset between strings to the get the SD and ES numbers as well. This is without the suppressor, no keyholing at 100yards the 10.6 and 10.3 grouped the best all within 1.5 inches for me.

    Gun: DPMS GII Recon 16inch Factory Barrel
    Hornady Brass TTL. 2.005
    Flash Hole Resized
    Pockets Uniformed
    FL Resized and Neck Resized to: .332
    COAL 2.790
    178grn Hornady ELX Match
    CCI Large Rifle Primer
    Measure 10ft From Barrel to Chrono

    Temp: 83
    1022.5 Baro Pressure
    27.4 Wet Bulb
    26.9 DP
    88.5 Rel Humidity
    -245ft Altitude

    10.6grain Trail Boss

    1077
    1028
    1059
    1034
    1071
    Avg Velocity: 1053

    10.3grn Trail Boss
    1077
    1025
    1059
    1034
    1071
    Avg: 1053

    1052
    1049
    1068
    1055
    1054
    Avg: 1055

    10.0grain Trail Boss
    1025
    1026
    1040
    993
    1017
    Avg: 1020

    1012
    1012
    1028
    1014
    1028
    Avg: 1018

    9.7grains Trail Boss
    983
    990
    998
    981
    992
    Avg: 988

    991
    996
    978
    986
    1007
    Avg: 991
     

    Moorvogi

    Firearm Advocate
    Dec 28, 2014
    855
    hehe giggle worthy fun. great for teaching kids to shoot a "big rifle" and not develop a flinch. Best part is, at 25 -100 yards on steel, they will feel great joy/success.
     

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