6mm BR subsonic?

[ODIN]

I am currently doing the obligatory research necessary for building a short barreled 6mm BR rifle.

one possibility I want to investigate is the feasibility of a heavy bullet subsonic load. I am well versed in the dangers of subsonic load development, but rather than reinvent the wheel, I was hoping to find someone who was successful and willing to share.

If I determine it can't be SAFELY done, then I will opt for a faster twist, and shoot lighter bullets through my suppressor. I just won't be as quiet.

Bob

 

[ODIN]

I did it today. !!!!!

1139 fps using 7.5 grains of Trailboss, pretty close to 90% of a full case.!

I used a 105 gr AMAX in my 18.5" 1 in 8 twist PacNor barrel.

On an additional note, I had the free bore reduced as much as possible when I built this barrel in order to take up case volume with the bullet. The base of the bullet sets about 1/16' below the widest part of the shoulder. Well today it all paid off. I now have to go shoot for group.

Stay tuned for further updates.

Fired through the suppressor all I could hear was the bullet hitting the target. I can't wait for the two coyote presentation. He He

I spent quite a bit of time talking with the ballistition at IMR (the guy that actually developed ""TrailBoss") Trail Boss was developed to fill up the cowboy action cartridges and still have reduced velocity for there safety parameters.

Bob

 

[camac]

Great work Bob, I heard of the 30BR boys doing this and pumping out the big (high BC) projectiles at low speed - spin was the issue so let us know how they stabilise in 8 twist. - sounds pretty awesome either way.

 

[Obtunded]

Not scientific, and not a direct answer to your question, but I have been shooting at 1-mile gongs with a 6BR and Berger old VLD's going 2900. By my calculations, they are just going subsonic, and with the carboard I have used, that transition to subsonic has not been an issue.

 

[camac]

Obtunded, thanks for the response. There is a difference though. Projectiles slow in velocity far faster than they slow in spinning speed. So a projectile started at 2900 fps and a 1 in 8 twist is spinning at around 261,000 rpm. By the time it slows to subsonic it should be still spinning at well above 200,000 rpm. Conversely a projectile started at 1139fps in 1 in 8 is only spinning at 102,510 fps - less than half the revs, and no where near as stable. I plugged some figures into JBM's calcs though and it might be OK - just interested to know??

 

[ODIN]

Well the weather was great so I headed to the range today.

The subsonic load was fun to shoot, but the best I could do with the 105 gr AMax was about 2", and 3 out of the five holes were keyholed.

So it looks like I have to start working my way down in bullet weights.

It was really cool looking through the scope at 20 power and watching the bullet for the last 50 yards, fall in a large arc and then hit the target.
Bob


When I get new results I will be back.
Bob

 

[camac]

The 100 Gr hornady round nose might work.

 

[ODIN]

I'm sure that I could make them subsonic. I think that the 1:8 twist is slow for any subsonic 100 gr. to be accurate.

I feel confident that with this Trail Boss Powder I can safely get the 87 gr to go subsonic.
Bob

 

[camac]

Hi Odin - The 100 gr round nose is only .950 long and will stabilise in a 12 twist when an 87gr hornady vmax won't.
BC is hopeless but then you won't be trying 1000 yards with it so might be acceptable. WIll have more thump.

 

[camac]

Just plugged the 100gr round nose into JBM's stability calculator and came out very stable at these speeds and twist (2.2 Miller stability factor - that is off the chart). The 87 Gr were pretty good too with a 1.5. Compared to just over 1 for the 105 AMAX. 1.0 is borderline, 1.5 is very good and 2.2 is outstanding.

 

[UK]

HI ODIN, WHEN YOU TRIED SUBSONIC FOR GROUP WERE YOU SHOOTING WITH YOUR SUPRESSOR? WE HERE IN THE UK USE SUPRESSORS ON ALL OUR RIFLES AND SHOOTING BULLETS ON THE EDGE OF STABILITY CAN WRECK YOUR MODERATOR NOT TO MENTION BEING VERY DANGEROUS TO YOU, EVERYONE HERE IS PLAYING WITH TRAIL BOSS AT THE MOMENT, JUST TRIED IT IN A 308. WHAT IS THE HOLE DIAMETER IN YOUR MODERATOR RELATIVE TO THE BULLET DAIMETER?

 

[camac]

Good point UK

 

[ODIN]

Thanks Lynn
I often wondered if that would work but never had the nerve to give it a try.

I had this suppressor custom built for 6mm rounds. The exit hole is .280 for us yanks, or 7.2mm for you would be soccer players ;D.

I won't be headed to the range for more developement for awhile, I fish off shore here in Oregon, and Salmon started last weekend and the tuna will show up shortly. This retirement thing makes it hard to schedule all the fun.

I still can't get over looking through the 20x scope and watching the bullet for the last 50 yards. Fun stuff.

Bob

 

[UK]

Hi Odin, dont know much about soccer or football I'm afraid.
The reason I asked about the supressor is that may it cause groups to be bad which would be ok if NOT shooting with the supressor.
Were the 2" keyholed groups with the supressor or not?

 

[ODIN]

All round were fired through the suppressor.

I have all my rifles threaded for this suppressor. The 6 BR , the .243 Ackley, and my Ruger 10/22 LR.

The 6BR is an 18..5" heavy barrel I had Pac-Nor build just for this subsonic 6mm adventure.

Bob

 

[UK]

I would suggest you shoot all groups without the supressor, when a long for calibre bullet is shot through a Mod (what we generally call a supressor in England) it can do very strange things to the grouping, then work down to the heaviest bullet you can shoot throught the mod and get good groups. With subsonic ammo the difference a moderator makes is really minimal anyway. Sound degrades by the inverse square law which means if you are 4 times the distance from the muzzle you hear 1/16th of the sound at the muzzle not 1/4, you will be amazed if you stand 100 yards downrange of the muzzle at 90 degrees to the shooter how little you hear, with the mod on or not. One of the main benefits to hunting with supressors is that the animal cannot distinguish the direction of the sound, and so others will hang around confused and you can improve your bag. No matter how quiet you make a gunshot, the impact noise will always disturb other quarry anyway.

 

[UK]

Lynn, thats a really interesting point (pardon the pun), I thought the bearing surface was what required an increased spin rate to stabilise the bullet, this would not change of course if the bullet is fired base first, maybe its not that simple. Have you tried firing a bullet backwards that will not stabilise in the correct orientation in a particular gun, does it work?

 

[RonAKA]

I see Hodgdon lists three loads for the .223 that are subsonic using Trailboss, Titegroup, and Clays powder. They also list a couple for the 308.

http://data.hodgdon.com/cartridge_load.asp

 

[camac]

I think the main reason the backwards works is you actually decrease lift caused by the aerodynamic shape. It is still a long bullet and therefore minimally stabilised at that revolution. But the opposing lift forces are greatly reduced. Personally I would still go for a round nosed. As Lynn said - it is no longer a high BC bullet anyway.

 

[ODIN]

;D
Well Tuna, and Salmon, season are pretty much over here in Oregon so I dusted off this old project and am happy to report I have a winner.

To bring you all up to date
The gun was built strictly for some subsonic action
A Pac-Nor 1:8 twist barrel at 18.5' long
Threaded for a 6mm special built suppressor.
The Cartridge is a 6mm BR Lapua
The bore was set very short in order to take up case space with the seated bullet. OAL 2.222"

The best load has been 87 Gr Berger, 5.6 Gr Trail Boss, 1043 fps.
one hole groups at 50 yards. (I ran out of prepped cases before I was able to do distance testing)

When I get the results for 100 yard groups I will post accordingly.

Let those coyotes walk past my kitchen door now!!!

Bob