Subsonic .308 Load, testing flash hole diameter

I've been playing with trying to find an accurate, repeatable, subsonic load for my little Remington 738. It's supposed to be a host for a new 8.6 BLK, 6 twist barrel but it's so freakin quiet with the .308 sub loads I may have to buy another one to cannibalize it. Or get a much better made Ruger in .308.

"Accurate" isn't something I've seen discussed when it comes to subs and for good reason. I'm working in the 50-100 yard range with an eye on 200 yards as there is still plenty of energy left at 200. I need to do more testing but the initial results were promising. It's stupid quiet with the Nomad 20 I'm running.

I started with the well known Trail Boss / 190gr Sub-X / ream the flash hole to 9/64" load. I got to wondering if I could improve the dismal accuracy I was seeing by adjusting "something". In supersonic loads something would be powder charge and seating depth. With the Trail Boss load there was flash hole to adjust too.

Changing more than one thing will often lead down a deep rabbit hole. For that matter, changing one thing seems to lead down a deep rabbit hole

Enough blathering. For my little rifle, can on, heres the first round of results for powder testing. I may will retest this. The flash hole in this test has not been changed from factory. Ie I didn't drill it out.



Here's the flash hole diameter test. 9/64" was not the best choice from an ES perspective. I forgot to add the date on this test. 08/30/24. 0.1094" has the best ES but velocity was a little lower than 10.7gr was with the factory flash hole diameter.

I guess someone is going to want to know how I opened the flash hole up to "4 digit precision". That's fair. It's wobbly precision and I seriously doubt that that the 3rd decimal place, much less the 4th, matters. I don't have my mini-lathe anymore but I do have some very small chucking reamers. I held the brass in a K&M Precision collet and holder, put the reamer in my handy and long lived Sinclair turning widget, and hand fed the case into the reamer. It was kind of a pita. All of them had to be reamed to .0938". Then 2,3, and 4 were reamed to 0.1094". Then 3 and 4 were reamed to 0.1250". And then 4 was reamed to 0.1406" (9/64"). I'm kind of curious what the ES looks like on either side of 0.1094" but I think I'll make sure it repeats for a larger sample before do that test.

The numerologists out there will carp on the fact the 5 shots are statistically bs and to that I would say - True. Why don't you do the test and burn up your powder and bullets. I'll keep trying to figure out a good cold bore load that hits where I want it to with reliability.

Cool. Thanks for the info.

What twist are you running on a sub .308?

FireFlyFishing said:

Cool. Thanks for the info.

What twist are you running on a sub .308?

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The internet says 1:10. Remington... it's not the same company it was.

Berger says that's marginally stable for their 190gr bullet. Obviously their bullet is going to have a better G1/G7 than Hornady's Sub-X bullet so the Sub-X probably wouldn't be as stable. I shot it at 200 yards a few days ago and the "group" was about 6". I was doing the flash hole test and only looking for velocity on the chronograph. I'll retest at 100 soon.

Apparently Trail Boss is hard impossible to find so I may experiment with something else. It's supposed to be coming back but I'm not optimistic. I still have quite a bit left so I have enough to keep testing.

Have you have better luck with a 300 BO in the accuracy/precision realm?

Not really but I haven't put a lot of effort into either. Buying the .308 wasn't intended to become a .308 sub project. I was going to take the new barrel off and order an 8.6BLK 6 twist barrel. But, I just had to shoot it when it came in. It's a compact little rifle that would be easy to hunt hogs with. So I ordered David Tubb's final finish bullets to clean the barrel up. New barrels are dismal looking when you borescope them. The Tubb kit cleaned the barrel up pretty nice. Then I shot it with supers and got to wondering about subs.

Other subsonic powders suggested here.

I'm interested in your journey. I have an extra .308 laying around and have been noodling what load to run through it. A sub .308 is intriguing.

I have a 300 BO and have dabbled with subs only once and have not returned to the project as work and a family gobbles my time, as well as serious hunting projects for my western big game adventures.

Mike6158 said:

Other subsonic powders suggested here.

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I just looked on their load data website and they have more powders listed there for a sub .308. You may consult that data as well. I noticed Benchmark is listed. That's a powder I've seen on the selves lately…

Work and family does gobble up time but it's worth it. I got to watch two of my grandsons (age 6 and 11) play football last night. They live about 90 miles away so it was a late night but I wouldn't trade the time for anything.

I'm really curious about what I could get with an increased twist rate barrel. Yet another rabbit hole... The Remington and Ruger line of rifles are easy to do barrel swaps on. I think the Ruger is better made. I'll keep working on the current rifle.

BTW - When I was at the range a few days ago I shot 5 rounds through it and a Dead Air Nomad 20 can. I stopped to check the barrel for temperature and the other 4 people that were shooting down at 100, 50, and the pistol range were standing there watching. One of them wanted to know what caliber I was shooting. When I said .308 they all looked at me with that yeah right look on their face. When I showed them the cartridge they couldn't believe how quiet it was. They thought it was one of the gee whiz pellet rifles they make now. It's ridiculously quiet. I think bullet impact on steel at 200 yards makes more noise.

I got about 3/4 moa with 308- 220g rn subs using green dot --- some people don't like using small amounts of fast powder but I've never had issues-- I use mag primers and drill flash holes

Following this. I am currently in pretty much the same quest in 308. I am using Titegroup and have been getting really good groups EXCEPT for the occasional vertical flyer. Occasional is about 1 shot out of every 5 shot group (at 100 yds). I'm running these through a chrono and I know that the flyers are lower velocity than the others. The question is what is causing the occasional low velocity round.
I am currently testing neck tension as the culprit. I will probably shoot those in the next week or so and be able to report on it.
I spent most of last year working up the quietest, most accurate 300 blk sub load. I am using 190 subx for both 300 and 308. I ended up at 6.2 grains Unique. However, my rifle lineup changed some when I got the 308. Since I have CFEBLK to use up, I am loading my subs with it. It is not as quiet, but got to use the powder for something.

By the way, I've found that differences of 50 or more fps in MV on subsonic loads makes a BIG difference in point of impact at 100 yds or more. My big challenge is identifying what is causing the variable ES and correct it. I use a hand press to seat the bullets and noticed a difference in the feel when seating bullets. Have started marking them so I can identify when I shoot them.
For the 308 I have neck sized some brass with Lee collet die and that gives pretty consistent feel when seating bullets.
For 300 blk, I annealed a bunch of brass and used a FL sizing die with expander removed, then expanded with mandrel. Haven't tested them yet.

Awesome! The more testers the better! I haven't done anything with neck tension or seating depth yet.

I've got 10.7 gr of Trail Boss loaded with .1094 flash holes. Sometime this morning I'll load 10.8 and 10.9 with the same flash hole diameter. The last test, with the best ES was a little slower than I'd like. We'll see how it goes.

.1 grain increments is pretty small but the % change in the load is almost 1%. Compare that to a .2 gr increase for a powder test in a 7mm Rem Mag charge (61.3 to 61.5) which is only .3%-ish. Obviously there's a lot of difference between the two powders and all of the chamber / barrel dynamics that go on so I'm not suggesting loading to second decimal place precision, just noting the difference.

I have a 1:9 twist 18" McGowan #5 contour stainless barrel on my tikka that shoots the 190 subx with star line brass cci large rifle primers and 8.0 & 8.4 grains of titegroup accurately to 150 yards. 8.0 grains in the winter and 8.4 in the warmer months. Benchmark worked well too but would have a flyer every time through various powder charges.

I'm going to load some up for my 1 in 10 Bergara with the 190 Hornady. I had not read about the flash hole, I thought about it when watching the UR video.
UR Sub 308

Comanche41 said:

Following this. I am currently in pretty much the same quest in 308. I am using Titegroup and have been getting really good groups EXCEPT for the occasional vertical flyer. Occasional is about 1 shot out of every 5 shot group (at 100 yds). I'm running these through a chrono and I know that the flyers are lower velocity than the others. The question is what is causing the occasional low velocity round.
I am currently testing neck tension as the culprit. I will probably shoot those in the next week or so and be able to report on it.
I spent most of last year working up the quietest, most accurate 300 blk sub load. I am using 190 subx for both 300 and 308. I ended up at 6.2 grains Unique. However, my rifle lineup changed some when I got the 308. Since I have CFEBLK to use up, I am loading my subs with it. It is not as quiet, but got to use the powder for something.

By the way, I've found that differences of 50 or more fps in MV on subsonic loads makes a BIG difference in point of impact at 100 yds or more. My big challenge is identifying what is causing the variable ES and correct it. I use a hand press to seat the bullets and noticed a difference in the feel when seating bullets. Have started marking them so I can identify when I shoot them.
For the 308 I have neck sized some brass with Lee collet die and that gives pretty consistent feel when seating bullets.
For 300 blk, I annealed a bunch of brass and used a FL sizing die with expander removed, then expanded with mandrel. Haven't tested them yet.

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A buddy of mine who shot a lot of subsonic rounds at crop damage deer (all head shots) swore by the Lee Collet Die for consistent neck tension and a light crimp with the Lee Factory Crimp Die for uniform "start pressure"- if that's the correct term.

It was so long ago when he first started doing it that he computed drop tables longhand using formulas with a pencil and paper.

Another thing that he emphasized was that it was important to fire form your brass to the rifle with full power loads before using the brass to load subsonics. Loading subsonics in virgin brass just doesn't produce enough pressure to fully obturate the brass and he never got good results that way.