7.62 X 39 subsonic BR round?

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model14

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Do you think subsonic centerfire benchrest experimentation is a futile pursuit? If not, it would seem the 7.62 X 39 cartridge would have potential (small case capacity, heavy, high BC bullet). Just curious what you think. Subsonic certainly works in 100 yard rimfire BR. An 1100 fps, 180 grain VLD would certainly have a much better BC than a 30 grain .22. Don't know how well it could be stabilized though.
 
Nothing ventured..

model14 said:
Do you think subsonic centerfire benchrest experimentation is a futile pursuit? If not, it would seem the 7.62 X 39 cartridge would have potential (small case capacity, heavy, high BC bullet). Just curious what you think. Subsonic certainly works in 100 yard rimfire BR. An 1100 fps, 180 grain VLD would certainly have a much better BC than a 30 grain .22. Don't know how well it could be stabilized though.
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Nothing gained. But I would not be surprised if you find that you are barking up a VERY LONELY tree.

40 grain round nose lead projectiles going 50 yards are LOT different from jacket bullets of any weight going 100, 200, or 300 yards

IMHO, FWIW

David
 
OK, random thoughts from a know-nothing:

My understanding is that the 22 RF Match is subsonic so that it never experiences transonic issues and that in CF the way this is done is to drive the bullet fast enough that it is still supersonic at the target.

I also don't think that the case will be the right capacity for subsonic velocity. Several folks are shootng what amounts to 7.62x39 "improved" (30PPC, 30 Major) at near 3,000fps. Leads me to believe that you'd have a hard time finding a powder that fills it up yet yields around 1/3rd that velocity.

Coming at it from another direction, the 300 Whisper is a 221 Fireball necked up to 308 and is subsonic with really heavy bullets...200+ grains. Don't know if the case is full even then. Off the top of my head, I think the 221 has less capacity than the 7.62x39.

FWIW, Greg J.
 
Aside from the fact that nobody shoots 30 gr .22 bullets in centerfire BR, and no matter what the BC of a 180 gr .30 cal bullet might be, a 66 gr 6 mm at 3400± fps would likely blow by the slow heavy bullet.

A 180 gr .30 cal could be stabilized at 1100 fps, but it'd need a pretty steep twist I'd imagine. Most .308's are using 1 in 12 or 1 in 10 twists depending on bullet weight and shooting discipline. Keeping a faster twist clean enough to shoot with the smaller bores might be all the fun a guy would want, or could stand.
 
I am ordering a 1:7 Douglas barrel to make a subsonic 7.62x39. Not to shoot BR, but to shoot critters quietly. I plan to launch 240 SMK's at 1100 FPS. I have read this is akin to a "flying drill bit." We shall see. Also chambering a 308 barrel for it too.
 
Just for grins, here are some comparisons . . .

model14 said:
Do you think subsonic centerfire benchrest experimentation is a futile pursuit? If not, it would seem the 7.62 X 39 cartridge would have potential (small case capacity, heavy, high BC bullet). Just curious what you think. Subsonic certainly works in 100 yard rimfire BR. An 1100 fps, 180 grain VLD would certainly have a much better BC than a 30 grain .22. Don't know how well it could be stabilized though.
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This concept isn't as crazy as I expected it to be! :eek: For comparative wind-drift, I used two ten ogive BIB bullets: the 118 Gr. vs.187 Gr. - both FB versions. At sea leve, standard conditions, and 1050 FPS MV, the 187 should be matched with a 1"11.6" twist: Sg = 1.5 (Note: above 20 Deg. F., a 12" twist would probably work just fine, imparting Sg close to 1.4). At 3000 FPS, the 118 needs only a 1:18" twist. Using a ten MPH cross-wind, 200 Yd.[to the target], and these MVs, , the 187 would drift 1.9 MOA, or, 4.1 inches; the 118 would drift 1.7 MOA, or, 3.6 inches! Not near the advantage I expected for the lighter bullet. :eek: For now, I'll stick to light and fast. .:D
I didn't make time to calculate relative recoil. RG
 
I have a friend that shoots an AR10, the 308 model. He has gone to the trouble to get a supressor fitted to it (all legal). He shoots a 170 grain bullet at about 1100 fps using 9 grains of IMR Trailboss powder. It fills the case about 3/4 or a bit more full. Barrel is standard 10 twist. He says it shoots just fine. Makes less noise that a 22 RF. I calculated the bullet energy and it is in the low 45 ACP range. But good enough. He just recently killed two deer, both head shots and mostly uses it to hunt hogs. Says it is so quite he can get in several shots before spooking them. He buys the loaded ammo from the smith that set him up with the supressor. I personally broke down one round to check the load as I was curious as to how they were getting the low velocity in that large a case. Trailboss is just the ticket.

Donald
 
Thanks for the informative answers to my question about a subsonic 7.62 X 39. Some people see the usefullness in being curious, others do not. It is what makes this sport so interesting to me.
 
You might consider a smaller case. In IHMSA alot of shooters are using 32-20 and 30 carb. at bullet weights and velocities you are looking for. I use a 175 grn. cast from a 10" TC at 1350 fps. with a 30 carb. I have shot a 200 cast at 100 yrds. It is a 1-10 twist.
Mugs
IHMSA 5940L
 
subsonic 50 for score

R. G. Robinett said:
This concept isn't as crazy as I expected it to be! :eek: For comparative wind-drift, I used two ten ogive BIB bullets: the 118 Gr. vs.187 Gr. - both FB versions. At sea leve, standard conditions, and 1050 FPS MV, the 187 should be matched with a 1"11.6" twist: Sg = 1.5 (Note: above 20 Deg. F., a 12" twist would probably work just fine, imparting Sg close to 1.4). At 3000 FPS, the 118 needs only a 1:18" twist. Using a ten MPH cross-wind, 200 Yd.[to the target], and these MVs, , the 187 would drift 1.9 MOA, or, 4.1 inches; the 118 would drift 1.7 MOA, or, 3.6 inches! Not near the advantage I expected for the lighter bullet. :eek: For now, I'll stick to light and fast. .:D
I didn't make time to calculate relative recoil. RG
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Very interesting. Using Hornady's calculator for 70 degrees temperature, a 0.308 110 gr Vmax at 3000 fps drifts 4.2" at 200 yds, the 0.308 208 gr Amax at 1050 fps drifts less at 3.5", and the 0.510 750 gr Amax at 1050 fps just 2.2". Muzzle energy is also reduced: from 2198 to 509 and 1836, respectively. For score shooting, a subsonic 50 BR looks good - half the wind drift with slightly less recoil than a 30 BR - and a BIG hole.:eek:

The kicker is that the subsonic rounds drop about ten times as much. ES of 20 fps gives over an inch difference in drop at 200 yds with a 100 yd zero for each. While this problem could be addressed by a combination a very low ES and barrel tuning in good conditions, head and tailwinds would cause much more vertical than we are used to, though still only about a quarter of the crosswind effect.

Cheers,
Keith
 
I was not deriding your

model14 said:
Thanks for the informative answers to my question about a subsonic 7.62 X 39. Some people see the usefullness in being curious, others do not. It is what makes this sport so interesting to me.
Click to expand...

curiosity. What I was stating that you might well be barking up a lonely tree. You did state "centerfire benchrest" not "quiet critter shooting" BIG difference! And even at that, unless you are COMMITTED to a 7.62x39 chamber, there are sure as heck a lot of more compliant sub-sonic cartridges out there. The "whisper" is the first one that comes to mind. It is even used by Arborists for deer predation.
 
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