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View Full Version : Subsonic 308 reload experiment #2


Artful
March 16, 2007, 08:55
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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 w/ 150 grain Hornady Flat Base spirepoint bullets fired by CCI 250 magnum primers. (a shorter bullet than the last bullets experimented with :wink: )

Weather 87 deg F - sunny, not much breeze... AAC Cyclone can

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).

13 grains of Trailboss
All fed from the magazine...
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

Now we had noticed an unsettling velocity gap that had developed in some of the strings and we decided to take a side branch to see why there could be several hundred feet per second spread in these carefully weighed powder charges.... deduced that the angle that the charge was in the cartridge case was playing a part in the velocity changes so to test that theory, that we tried loading ammo into the rifle in different ways.

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.

first we just wanted to see if the up tilt was going to produce a difference

12.4 Grains of Trailboss
Can off - Grenade gas Setting - up 929.2 , up 1079, up 1146
Can ON - Grenade gas setting - up 929.0, up 1016,
Can ON - Grenade gas setting - bolt 1324, bolt 1311,
Can ON - Grenade gas setting - up 1115

12.2 Grains of Trailboss
Can off - Grenade gas setting - up 986.2, up 1120, up 1148, up 992.2
Can ON - Grenade gas setting - up 951.9, up 1117, up 1185, up 1145

12.0 Grains of Trailboss
Can off - Grenade gas setting - up 956.1, up 960.0
Can ON - Grenade gas setting - up 1087, up 1141
Can ON - Grenade gas setting - bolt 1282, bolt 1281, bolt 1268, bolt 1247

11.8 Grains of Trailboss
Can off - Grenade gas setting - up 950.6, up 1170
Can ON - Grenade gas setting - up 1024, up 1147
Can ON - Grenade gas setting - bolt 1261, bolt 1220, bolt 1232, bolt 1208

All holes in the paper target exhibited roundness with no tipping evident, but several times it was noticed a wide swing between impact area's with can on vs off and again between tilted and bolt loading but I would expect this with velocity veriations at these slower velocities.

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

Conclusions:
While lubing will help with subsonic loads, I'm not sure it's required.

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)

While case loading density is about 1/2 the case, I'm not yet sold that Trailboss is the best choice for subsonic loading.
_________________
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Azrial
March 16, 2007, 13:37
I am no master reloader but I think that you are on the money, case volume vs charge weight is the culprit. In strightwall cartridges, the answer to that problem is a cardboard or Styrofoam wad to keep the charge near the primer.

Have you also considered using a heaver weight bullet? I have some 190 Sierra BTHP's I have loaded in .308 Win.

Artful
March 17, 2007, 00:31
Originally posted by Azrial
I am no master reloader but I think that you are on the money, case volume vs charge weight is the culprit. In strightwall cartridges, the answer to that problem is a cardboard or Styrofoam wad to keep the charge near the primer.

Have you also considered using a heaver weight bullet? I have some 190 Sierra BTHP's I have loaded in .308 Win.

Not yet, I'm trying to get bullet weights that are inexpensive and common before I play with more expensive bullets.

Next week I'll be doing comparison between TRAILBOSS and some old stand by's like RedDot and Unique I think.

welding inspector
March 17, 2007, 01:33
Artful
I shoot a 338cal 300 grain sierra @1050 or so but I use a small case with a large bullet. As Azrial said a larger bullet such as the sierra 220 might help cut your case capacity down. Also finding a powder that fills the case more will help a lot you might try V V or Accurate.