308 subsonic questions

[JDT]

Hey guys,
So I've read and read over a period of time here don't know that I've been able to read everything as Im still not great with getting the search feature to find exactly what Im looking for but anyway here goes. First off I have reloaded subsonic .223 and 9mm but new to the subsonic 308. It will be shot out of a 1 in 12 24" FN SPR with 7.62sd suppressor. I would like to shoot at 100yrds with it. I have come to the conclusion that the trail boss is the powder of choice and a round nose bullet is preffered. I wouldn't mind knowing if anyone has loaded for this particular rifle what a good starting load is and what exact bullet and col. would be. I think 180gr bullet is what I was leaning towards but again don't know for sure. My main concern is somewhere I had read about the subsonic loads running a risk of having extreme pressures for some reason at some point not the initial 1st shot but after several shots or so. I couldn't remember if it was due to unburnt powder left in the barrel or what. If someone could please clarify this for me as I can't figure out where I read it and of all things I would rather not ruin a rifle if I can help it. Thanks in advance for the help I know you guys get tired of the same old questions being asked.

 

[Luke]

Re: 308 subsonic questions

http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=468230#Post468230

 

[JDT]

Re: 308 subsonic questions

Thanks for the link I had read that before and just read it again. So correct me if I am wrong but the pressure problems are due to such a low amount of powder in the case therefore running the risk of the primer not being able to ignite the powder sufficiently, since the powder could be laying down in the case and not in direct contact with the flashhole. Is that why some people drill out the flashholes. Am I on the right track with this or am I all wrong.

 

[Luke]

Re: 308 subsonic questions

You are correct, but as long as you use a high volume powder like Trail Boss you shouldn't have any problems.

I was just running the numbers for my pesonal loads and discovered this:

The .308 overall case capacity is 56 grains of water = 3.62874 grams = 3.62874 cubic centimeter. My own load is 10.5gr of Trail Boss * .2172 (VMD of Trail Boss) = 2.2806 cubic centimeters. So my load fills up ~62% of the case, not including the space occupied the bullet, which should push it up above 75% of the case filled, which SHOULD give you reliable, safe ignition. As with all things reloading - your mileage may vary.

 

[JDT]

Re: 308 subsonic questions

Ok now that I have that clear I was going to try the 180gr rn hornady but I read somewhere that the 1in12 twist may like a 150 gr bullet for subsonic speeds. Any truth to that as most of you guys seem to be using at least 178gr bullets. This will be mainly for punching paper so not as worried about a heavy bullet carrying a heavy punch down range. More concerned with accuracy.

 

[CARRYAGLOCK]

Re: 308 subsonic questions

In my FN SPR 1in12 20" I use the Trailboss load with a 180gr Hornady Round Nose .30-30 projectile. My FN is a dedicated suppressed rifle for me and loves this load.

You'll have to work down the load but mine is perfect somewhere between 10grs and 9grs I don't remember but you have to start high like 12 grs and work down by .5 grs increments. When you go too low and your impacts are oblonged slightly, then work up by .2 grs to the peak accuracy spot. You can go .1 gr increments instead at this point if you want.

 

[JDT]

Re: 308 subsonic questions

Thanks that really helps me out, I know that I have another 4" of barrel length but that gets me a good starting point. Do you remember the col that you were using with the hornady 180gr rn.

 

[CARRYAGLOCK]

Re: 308 subsonic questions

It has a cannelure and I seated the bullet so the case mouth rim is just into the cannelure although I do not crimp subsonics. That's a whole different animal I haven't needed to tackle.

With playing with the charge alone I got 1moa accuracy with my pet load.

 

[Fritzcat]

Re: 308 subsonic questions

I have been using 10.5gr TrailBoss with Horn 180gr RN and 1x10, 20" Shilen. Can shoot MOA and better. Work your load and check the paper for stability before you put the can on. I have a AR 7.62x38 with 1x10, 16". It stablizes the 220gr Horn RN. I wanted heavy for hunting and to cycle the action. Last week I finally got to pull the trigger on a 150lb. Hog. He went about 80yds.

 

[CARRYAGLOCK]

Re: 308 subsonic questions

+1 on doing your load development tests on paper for oblonged holes BEFORE attaching the can!

But with the 1in12 twist barrels you (JDT) and I have 180 is the heaviest the 12 twist can stabilize moving a subsonic speeds.

In my 1in10 I like 220grs, Big difference that twist rate makes.