Loading Subsonic/Accuracy Expectations?

[Nostradumbass]

I’m working a load for a custom built .300 AAC Remington 700. The barrel is a 16” Bartlein 1/7 twist.

I have found some inconsistencies with muzzle velocity, and I’m working hard to understand the problem.

I suspect the issue may be with neck tension, and inconsistencies between neck wall thickness and brass hardness.

I separated my brass and loaded all same headstamp brass with 220gn Berger Hybrids, and 10.5gn CFE BLK. Projectiles were set .010” off the lands. With this load, I was able to manage barely 1 moa five shot groups. I may also try them kissing the lands as well.

Using brass made from 5.56 lake city cases, the best I can do hovers around 2 moa.

I have since ordered a few hundred 223 Lapua cases, neck turning lathe, neck sizing die, as well as a Redding bushing die.

Please let me know if you guys think I’m off track. The reason I think neck tension is the issue is because of the variable resistance I see when seating projectiles. Is there anything else I can do to ensure consistent neck tension? Would crimping be a good option? I just hate to crimp for a bolt gun.

And, of course I could be going down the wrong rabbit hole here..... I’ve tried multiple Bullets and found that the Berger 220gn hybrid and Sierra 220gn bthp shoot the best so far....so I’ve been trying to develop a load with those two.

Thanks so much in advance!

Happy New Year!

 

[db2000]

This is an 8” Seekins with subsonic at 25 yd.
The 10 shot group is Nosler 190 HPBT and A1680.
The 4 shot group is Hornady 190 Sub-x and A1680.

 

[Dead Eye Dick]

We built a beautiful custom XP-100 in 300 Whisper. The gun was everything we expected it would be. Flattened steel rams at 200 meters like they were hit with a sledge hammer...When it hit the rams. Both Brenda and I are international class shooters so when we took aim at a ram, we expected to hit it.

Velocities for the Subsonic loads were all over the place. We tried every thrice, but SD’s were quite high. Powders, load length, brass, bullets, nothing provided consistency. When a round is only going 1000FPS, a 100FPS differencebetween rounds wasvery significant.

 

[MALLARD]

I don't know anything, with that being said that powder has a burn rate similar to that of full load/full power powders for that cartridge, so i would probably look at a fast burning pistol powder. You can't just reduce the speed/charge weight of a normal burning powder and let that be your subsonic load. It doesn't work like that.

1 moa isn't bad

i don't see a level on your gun , maybe it's on the other side or you don't have one

neck turning is for tight neck chambers , if you don't have a tight neck chamber do not turn your cases, the additional case neck expansion is worse then poor cocentricity

crimping maybe, idk , some people say yes , it may depend on the amount of case neck tension you had, if your already doing .010 then i dont think crimping would help but if it was .002-.005 maybe

idk about berger hybrids, but the berger vld's have less than forgiving tangent/ogive and you need a chamber made to match that, the sierra's are more forgiving. but if your gun shoots both just fine i guess it wouldn' matter.

 

[Badfinger]

I've shot 220gr Berry's bullets using Accurate 1680 powder at 1144fps that hover around an inch out of a 16" Rainier Arms match barrel. Nice accurate and quiet load, I haven't tried to improve it any. I don't mod .223 brass I shoot Starline FWIW.

Good luck with your loading

 

[orkan]

I did a precision 300BLK some time back. It was a fun project.

Subsonic - 5 shots at 100yds with 10.5gr of IMR 4227, 175SMK's at 1045fps.

Supersonic - 5 shots at 100yds with 16gr of IMR 4227, 175SMK's at 1715fps.

The real issue with subs when using jacketed bullets is the first round inconsistency. The groups can get real tight and the accuracy very consistent after the first round is fired... but the first round will often land as much as half a mil away from the subsequent rounds. It's somewhat predictable, being typically +/- 0.1 mil away from the 0.5mil call... but very frustrating none the less when subsequent rounds are able to produce some ridiculous precision and accuracy. I've experienced this with 308 subs, 338 whisper, and every other subsonic load in every cartridge I've ever worked with when using jacketed bullets. I never had interest in working with cast bullets much, but I'm told that if you lube them correctly, this issue of first round impact shift can be solved.

I found 300blk was very effective as a training tool with the supersonic rounds, as with the crazy precision they offered... the slow bullets could really show imperfections in a shooters technique.

 

[Nostradumbass]

I did a precision 300BLK some time back. It was a fun project.

Subsonic - 5 shots at 100yds with 10.5gr of IMR 4227, 175SMK's at 1045fps.

Supersonic - 5 shots at 100yds with 16gr of IMR 4227, 175SMK's at 1715fps.

The real issue with subs when using jacketed bullets is the first round inconsistency. The groups can get real tight and the accuracy very consistent after the first round is fired... but the first round will often land as much as half a mil away from the subsequent rounds. It's somewhat predictable, being typically +/- 0.1 mil away from the 0.5mil call... but very frustrating none the less when subsequent rounds are able to produce some ridiculous precision and accuracy. I've experienced this with 308 subs, 338 whisper, and every other subsonic load in every cartridge I've ever worked with when using jacketed bullets. I never had interest in working with cast bullets much, but I'm told that if you lube them correctly, this issue of first round impact shift can be solved.

I found 300blk was very effective as a training tool with the supersonic rounds, as with the crazy precision they offered... the slow bullets could really show imperfections in a shooters technique.

Thanks Orkan,

Very good information. Was your barrel a 1 in 7 twist? Additionally, did you always use factory produced blackout brass, or did you make your own?

What is your thoughts on the first round being off zero? Could it be the friction of cold rifling on the slow moving projectile?

 

[redbullitt]

Between 1 and 2 moa at 100 yards with subs is quite good in my book, especially if it's consistent. 1 or just a little under consistently is a tough goal... but it is doable I think.

Cast bullets done right can indeed make this easier and more consistent,BUT it adds another set of variables for sure.

All the little things add up with subs, especially shooting form. Follow through makes a big difference for sure here. Handloading needs to be top tier level too. 10 fps really shows up here. Load and shoot like you're doing elr work.

I've used 7 and 8 twists. Otherwise did fine with jacketed bullets from 90 to 249g. The heavy 249g cast bullets were usually the best, generally 1.5 moa give or take a littlr at 100. Likely could be improved with better casting technique. there is a light 90 or 95 xtp that shot lights out in a couple different 300blk rifles with trail boss.

Used both converted and factory brass. Both good as far as I could tell once they were neck sized for the rifle. Annealing seemed to help notably.

With jacketed, The 208 a max performed well subsonic for me, as well as the 180 and 220 round noses. As far as powder, Lil gun was probably the best. H4198 showed promise too. Trail boss was exceptional with the light bullets. Without getting into cast stuff, you still have lots of options to try. Some may surprise you. One rifle really liked the flat nose 30 30 bullets and the rubber tipped hornady 30 30 bullets.

Take a little imperial sizing wax. Lube the jacketed bullets before you shoot them... you may be surprised. There is a guy who shoots old milsurps here with good success and he swears by it with reduced jacketed loads. Said it helps with vertical stringing. I've not messed with it enough to say for sure, but might be worth a shot if your rifle likes it.

Your group pic and orkans look fantastic. WAY better than what most people are getting subsonic. Note that most folks post pics from 50 yards in this type of application. If you get an honest 1.5 moa CONSISTENTLY with subs, you're way ahead of most other 300blk shooters.

 

[orkan]

Thanks Orkan,

Very good information. Was your barrel a 1 in 7 twist? Additionally, did you always use factory produced blackout brass, or did you make your own?

What is your thoughts on the first round being off zero? Could it be the friction of cold rifling on the slow moving projectile?

In order:

8 twist.

I used Lapua exclusively.

Bore condition... for sure. What specific thing? I'm not sure... I never have been able to quantify the exact cause. I never happens with supers... only subs. Doesn't happen in my 22lr's.

 

[orkan]

Your group pic and orkans look fantastic. WAY better than what most people are getting subsonic. Note that most folks post pics from 50 yards in this type of application. If you get an honest 1.5 moa CONSISTENTLY with subs, you're way ahead of most other 300blk shooters.

I suffered with poor performance by trying to follow the internet's advice on loads. IMR4227 was the key to unlocking the real potential of the cartridge for me. That, coupled with the use of 175SMK's... as they are super forgiving. Most other combinations put me up in that 1moa+ region you speak of.

 

[redbullitt]

I never had 4227, but I'm going to have to try after seeing that haha.

American select and a few other oddball powders did well, but I never got anything as consistent as your stuff. Here and there sure, just often enough to make me keep messing with it haha.

 

[Nostradumbass]

An update from my current load development.
I haven’t yet been able to tighten my 100yd groups much more than 1 moa so far.

I was very pleased with the chronograph results though. A ten shot group of 220gn Berger hybrids over 10.5gn’s of CFE Black yielded exceptional results.
Average MV- 1038 fps.
Extreme Spread- 37 fps.
Standard Deviation- 12.32 fps.