View Full Version : Subsonic 7.62 with Trailboss part 3

March 29, 2007, 16:49
I've wanted to load some subsonic ammo for my .308's, todays experiments concentrated on my Imbel FAL with it's 21 inch barrel. We had some more rounds to try out. These loads are not recommended and is only for information purposes. This is our day...

All ammo loaded in Remington 308 case's with flash hole expanded to 1/8th inch and 163 grain Pulled Military FMJ Flat Base spirepoint bullets that had been coated with Moly Disulfite dry lube from Bruno's fired by CCI 250 magnum primers. (same as first experiment :wink: )

Weather 76 deg F - sunny, not much breeze...
AAC Cyclone can wasn't used today

Last time we had taken Suputin's advise and cleaned and lubed
between groups of powder trials [http://members.shaw.ca/cronhelm/DevelopSubsonic.html],
today we wanted to see if we could use the barrel without special preperation
(which is how it would mostly likely be in the field).

first loading was
13 grains of Trailboss
Can Off - 936.3 1220
Can Off tilt up before firing - 1238 1112
Can On tilt up before firing - 1155

last time in experiment 2 with 13 grains of Trailboss with smaller normal flashhole it gave
Can off - Grenade gas setting 1251, 1332
Can ON - Grenade gas setting 1309, 1367, 1340
Can ON - Semi-Auto Setting 1243, 1336, 1317

12.8 grains of Trailboss
All Fed from the Magazine
Can off - Grenade gas setting 1181, 1202
Can ON - Grenade gas setting 1097, 1313, 1334, 1313, 1338, 1333

12.6 grains of Trailboss
All fed from the Magazine
Can off - Grenade gas setting 1163, 1199, 1089
Can ON - Grenade gas setting 1142, 1277, 1293, 1158, 1317

Today we got
12.5 grains of Trailboss
Can Off - 816.2 1142
Can Off tilt up before firing - 1087, 988.8, 1074

12 grains of Trailboss
Can Off - 1080 1070
Can Off tilt up before firing - 904.6, 815.1

11.5 grains of Trailboss
Can Off - 795.5, 940.2
Can Off tilt up before firing - 1145, 1092, 698.4

11 grains of Trailboss
Can Off - 597 768.9
Can Off tilt up before firing - 680.9 953.1

10.5 grains of Trailboss
Can Off - 256.2 , 690.5, 829.0 699.8
Can Off tilt up before firing - 494.3

We terminated the experiment at that point due to low velocities exhibited we had gone below the target range of speed we were looking for.

*The TILT UP method involves putting a cartridge in the chamber and then tapping the gun to settle the charge to the rear of the case then gently lowering the muzzle to the rifle rest and firing the charge.

The other method of loading the charged cartridges, was done by laying the cartridge on top of the magazine leaving the rifle facing the target and hitting the bolt release, which will distribute the powder more or less evenly over the bottom of the case as the BOLT Rammed it home.

NOT all holes in the paper target exhibited roundness with tipping evident, the 150 grain bullets used last time in standard FAL barrel do better.

Now keep in mind these where all individually weighed powder charges
so they are as listed to with accuracy of the balance beam scale and electronic scales.

While lubing bullet with Moly will help with subsonic loads (ie no sticking bullets) I didn't see large difference in it velocity wise.

Not even the hottest load attempted to cycle the action.

Evidently Trailboss is sensative to powder position (in fact moving the powder to the rear of the case seems to lower the velocity by several hundred feet per second with the same powder charge and I think I may have moved the powder to the front of the case inadvertantly before firing the 256 fps shot :shock: ) we were hoping that opening the primer flashhole up would eliminate this problem but it didn't.

While case loading density is less than 1/2 the case, I'm not yet sold that Trailboss is the best choice for subsonic loading. Still looking for better solution for reloading subsonics at home.

March 29, 2007, 22:44
I found a sub-sonic .308 recipe somewhere on line with 9.5gr of Unique and 150gr bullets. I'm going to try these in my .308 bolt. It may be worth trying in the FAL.

Would 2400 be a possible powder candidate? Our resident owl loaded up some 7.62x39 with 123gr bullets and 14gr of 2400 and he had good results. Probably not sub-sonic, but you could start at 14gr and work backwards.

I have read that low density loads with Unique do not seem to have any problems. The powder doesn't seem to be position sensitive. I have no idea about 2400 though.