308 subsonic

[Code Red]

Originally Posted by Elkman1

Artful, you seem to be knowledgable on this matter.
I'm looking for some direction from yourself or any other reader of this posting.

I have a DPMS 308 Long Range Rifle. The barrel is 1 in 10 twist. I have a Gemtech TPRS Sup. I am researching where to start in developing a sub-sonic round for my set-up.

I have read so many opinion in the last few days that my brain hurts. I have done alittle reloading myself but mostly pistol ammo and 22-250's. In my search, there seems to be a consensous that underloading (to much air) or using fillers (cotton fibers) is a bad idea. I did find an article where the auther suggested the use of dacron. (pillow stuffing) and claimed that it incenterated completely without clogging. I'm not sure why someone would like to take a chance and clog the baffles in a Sup...so for safety reasons, I have decided to start out using low volume 308 cases.

I don't know how the receipts will differ between pressures. For example one of my questions would be: Would the pressures be the same if I put 12 grains in an reduced volume case with less air vs. 12 grains in a standard underloaded case with more air. I'm looking for a safe starting point...Bullet weight?, Powder type?, Charge amount?, Standard/Magnum Primer. Any advise....?

And even a more important question, will it even be possible to get this AR-308 to even cycle rounds? If not, are there any safety concerns if I use this rifle as a single shot. Meaning, if there is not enough blow back to cycle the next round, are there any side affects that will cause damage to the gas tube, bolt assembly, etc. if I were to manually feed it one at a time?

Hey Elkman,
I hop that some of this may be helpfull for you. Artful is correct that if you don't have a modified gas system the bolt will not fully cycle, it will partly open, needing a lot more cleaning. I get between 15-20 rounds before I notice the bullets wont chamber as nice. If you send your upper to www.medeshafirearms.com they can instal an external bolt handle on your bolt carrier. This makes chambering your rounds a lot easyer than the T-handle. I had this done to my SR-25 just for subsonic ammo. I think it cost around $110. As far as a place to start for a load, I called Hodgdon (913-362-9455) they said to use 8gr Clays with 168gr Sierra Match and Magnum primers, no filler and Winchester Brass. I didn't have any Clays at the time but I did have a lot of Titegroup so I tried that with 8gr and it worked very well. I got some Clays and tried it, I found that I could not get the same accuracy with the Clays versus the Titegroup but the Clays was a bit more quiet. I found that useing 8gr of Titegroup with 173gr Military Match projectiles will produce sub MOA at 100m. I shot 20rds through my chrony today with a low of 1028fps and a high of 1062fps and an average of 1052fps. Temp was 48 degrees, humidity 40%, elevation 802ft.
I also spoke with a rep from Lapua, (630-350-1116) for a good projectile for sub loads. They have a 200gr FMJBT product code (B416) that they make specific for sub loads. They said that in a 1x10 barrel if you load them with the base of the bullet forward that they will stabalize. I bought some of them but have not tried this yet. I will post as soon as I do, should be next week some time.

Once again I hope that this is helpfull.

[Code Red]

I finaly got some of the Lapua B416s loaded and went to the range today. Over all length with the base of the bullet loaded forward is 2.474"
Temp was 42 deg. humidity 60% elevation 802ft. I loaded 14 bullets to start so I could find out what works.
Starting point was 9.4gr tite group. 1st shot was 1200fps 2nd shot 1188fps. 9.2gr, 1st 1166fps 2nd 1128
9gr, 1st 1089fps 2nd 1103,
8.8gr, 1st 1083fps 2nd 1074
8.6gr, 1st 1019fps 2nd 1025
8.4gr, 1st 1035fps 2nd 996
8.2gr, 1st 986.9fps 2nd 989

Suprisingly none of these showed signs of tumbling at 25yrds. All punched a clean hole through a 1/4" particle board.
The only problem that I ran into was the fact that with the flat base of the bullet forward I had to single load each round. They would not feed from the mag. I think that if I opened up the feed ramps than they might feed.
Any suggestions on this would be wonderfull.
I am going to load more up with 8.4, 8.5, and 8.6gr and go back to the range to get more data. I will load up 15 of each to get a good spread. Will post more when I get this done.
Any suggestions would be great.

[RWBlue]

bump, so I don't loose this thread

[js1050]

great info, I can't wait to start developing subsonic .308 loads.

[Artful]

I've wanted to load some subsonic ammo for my .308's
I started with the Evil Black Rifles HK91 and FAL my buddy brought his PTR91 (hk91 clone) and we had some 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/ 163 grain Flat Base spirepoint pulled military surplus bullets fired by CCI 250 magnum primers.

Weather 82 deg F - sunny, not much breeze... AAC Cyclone can

12 grains of Trailboss
my HK fired 1096 fps and 1061 fps in clean barre and oiled barrel then put
Hornady Oneshot Case sizing lube on bullet let dry - velocity 696.6!

My Bud's PTR91 fired in clean and oiled barrel 1093 fps and 1071 fps and 986.8 fps
then tried one with OneShot Hornaday case lube on the bullet 636.6 fps - tried another without OneShot on it 1024 fps

11.5 grains of Trailboss
My HK91 962.6 fps and 1016 fps
PRT91 994.9 fps and 829.7 fps
my FAL w/ 21" barrel 845.6 fps and 915.3 fps added OneShot 781.5 fps and 811 fps

cleaned and re-lubed barrels w/ CRC Power Lube
11 grains of TrailBoss
My HK91 785.1 fps and 726.8 fps
PTR 91 940.1 fps and 824.4 fps
FAL 856.4 fps first shot, 657.6 fps second shot, 567.0 fps third shot and 726.1 fps final shot

10.5 grains of Trail Boss
My HK91 636.7 fps and 762.5 fps
PTR91 691.8 fps and 769 fps
cleaned and lubed barrel on FAL w/ CRC Power Lube
FAL 793.7 fps 758.9 fps nice bullet holes figured I'll put the can on to see what it sounds like - 514.8 fps bullet perfect KEYHOLE in target - tried to shoot another and bullet lodged in the barrel yes did get it out - pulling all the lighter powder charges

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:
redo test with slightly higher powder charges and only 2 ten's change in powder charge -
That lubing the barrel doesn't keep you from sticking a bullet -
That shooting the bullets with Hornady OneShot case lube significantly slows the bullets down.
That somehow at very low velocity the addition of the suppressor caused the bullet velocity to be further retarded in the FAL - something I haven't seen with full power ammo.

[pug]

Artful interesting info. My test routine is to run an oil patch through then a dry one and fire a round. Then I start measuring the velocity and after 20 rounds I do the whole thing over. Your testing seems to confirm my rule of thumb that when the bullet drops below 750 fps you had best start looking for a stuck bullet or at least thats when they seem to start not popping out for me. Glad you could get the stuck one out. Couple of months ago I had to chop a sks because I wasn't so lucky but I got a nice parts rifle now.

[Artful]

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 )

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.

[rumlover]

308
10" encore 1/8 twist

240 SMK's with 12-15gr trail boss seems to keep me in the 950-1010 range. I just can't seem to get to the 1050 mark. Any suggestions?

[Artful]

Originally Posted by rumlover

308
10" encore 1/8 twist

240 SMK's with 12-15gr trail boss seems to keep me in the 950-1010 range. I just can't seem to get to the 1050 mark. Any suggestions?

You could try a hotter primer, or lube the bullets / barrel. Personally I'd be happy at consistant 1010 - 40 fps won't be noticed at the other end.