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Tried some Atomic .308 175 grain today. 1030 FPS Low noise and low recoil. Sound level is very low. But no matter what Piston I used bottomed out, action would not cycle. Under gassed. Basically a single shot weapon. Anybody else have any experience with Subsonic .308 in the RFB.
 

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Not a big surprise to me. From what I'm seeing normal 175gr loads are around 2600fps. That's more than 2.5x the velocity, 6.37x the energy.

I'm surprised they did such a light bullet. I see 300BLK subsonic as heavy as 240gr...
 

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Not a big surprise to me. From what I'm seeing normal 175gr loads are around 2600fps. That's more than 2.5x the velocity, 6.37x the energy....
You hit the nail on the head.
The handbook says the RFB will shoot subsonic single-shot (think bolt-action).
The RFB with it's heavy bolt carrier makes it the worst candidate to try to pull off cycling with subsonic ammo..... even with a suppressor.
There isn't even a fraction of the power needed to cycle it.

Here's what we did:

Took an RFB that happened to be overgassed (would cycle to mag-latch with gas valve cap removed) with standard ammo, no suppressor.

Screwed a suppressor on it.

Screwed the gas valve cap on it and closed it down to zero.

Shot subsonic ammo with 220 grain bullet.

It budged the bolt maybe an inch:rolleyes:.

Not EVEN close. Not gonna happen.

If we had shot that setup with standard ammo:eek:, we would have had lots of broken parts.

If you want to try to shoot subsonic suppressed, that's what the 300 blk is all about.
No sense in having a big .308 case that is only 1/3 full of powder (2/3 empty space)

Edit: It did sound like a paintball gun though:D.
 

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Heavier would certainly help, but only so much.

Lets say we start with the 175gr 2600fps load I mentioned before. If we compare that to a 300gr 1030fps subsonic load, we're still talking a factor of 3.7 in energy. Obviously it'll be more likely to work than a 175gr at the same velocity, but I still doubt it'd work well. I'll admit I haven't tried at all, though.
 

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The physics are complex. All I know is a 230gr .45acp recoils way harder than a 185gr despite the 185gr having more "energy." (I also believe "energy" is just a longstanding marketing tool, and that momentum is a better predictor of terminal effects. Energy however may be a better predictor of ballistics.)
 

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In that case we can look at momentum. 175 x 2600 = 455,000 (units don't make sense but doesn't matter). 300 x 1030 = 309,000. Still quite a bit lower but not quite so much.

I think somewhere between momentum and energy probably makes the most sense, I don't know where exactly though... mass x velocity ^ 1.5 or something?
 

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The physics are complex. All I know is a 230gr .45acp recoils way harder than a 185gr despite the 185gr having more "energy." (I also believe "energy" is just a longstanding marketing tool, and that momentum is a better predictor of terminal effects. Energy however may be a better predictor of ballistics.)
Energy is just momentum X velocity. Energy is what kills with a rifle, its how a tiny lightweight projectile does so much damage. Most rifle bullets weigh in at less than a 45 or 44 mag; many weigh in at less than a heavy 9mm! But they are travelling ~3x as fast. That energy transfer on impact is significant.

Recoil ... the bullets from factory are not normalized. That is, a 230 grain and a 180 grain are not set up to produce the exact same pressure and forces. This has nothing to do with the weight of the bullet but the selected recipe for the loads. If you kicked the 180 hard enough, it would recoil as hard as the 230 -- once their MA is identical. Whether that is a safe load or not depends on the caliber, I don't know about the 45, its doable in a 9mm thanks to the wide range of pressures and materials (a light +p can normalize to a heavy standard pressure load).

You need both. A thrown brick can equal the momentum of that 45, but it isn't likely to kill your attacker. A spitball shot at > 10k fps won't even break the skin past a very short range because of its mass to surface area and air resistance and breaking apart etc.

I don't see anything any heavier than 240s for 308ish size. I also looked at 303 (310 diameter) stuff and other near matches. What is the application? You can obviously load to match a handgun.. 240 shot like a 45 acp out of a 308 ... and it will still kill at 50 yards or so but now you are lugging around a gigantic pistol.
 

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As far as I can tell, 308 firing subs will have identical characteristics to 300BLK firing the same subs... if there's enough space in the 300BLK magazine to fit the bullet.

The longer skinnier bullet will have significantly better ballistics than a pistol cartridge but given the low muzzle velocity it'll certainly have a rainbow trajectory. I imagine it has plenty to still kill at 400yd, but you'll need to know range very accurately and I doubt many scopes allow enough drop.
 

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A 300gr+ bullet in 30 cal would be pretty damn long. Doubt it would fit. What would be the point? The point would be to make a single-shot rifle a semi-automatic rifle. Let's not miss the trees for the forest now :)
 

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I Think to get 300 you would need to make it closer to a cylinder than a standard shaped projectile. Not longer, but fatten the nose...
300 grains is a lot of lead.
Even if it were a cylinder with a flat front, it wouldn't get you even close.
The heaviest I could find were 240 grain and they are already REALLY LONG:
Longboard Metal Wood
Copper Metal Brass Bullet

The one on the top requires a 1 in 9 twist rate and the one on the bottom is 1.428" long.

If it was a jacketed lead round nose, consider 300 gr would be two of these 150 gr, back to back:
Bullet Ammunition Gun accessory Metal Copper

:eek:
Bullet Ammunition Gun accessory Metal Brass

:laugh2:
 

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Don't you also need faster turn rate at lower velocities, since that means lower rpm? I've heard of cartridges that'll stabilize out of 20" ARs but not 16".
Yes. And then there is that. For subsonic (which is less than half the FPS), I would assume twice the twist rate:rolleyes:.
...which would mean a different barrel...
and it would be easier starting with a different gun like oh, the 300 AAC Blackout.
Which was designed from the beginning (1 in 7" twist) to do this and still cycle.;)
220 grain 1000 FPS:

You can even get subsonic ammo with glowing tips:eek::
 

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Glow in the dark bullets remind me of Chad Pennington. His highly accurate throwing stemmed from the fact that his passes were so slow, he could run them down and correct them in mid-flight.
 

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Glow in the dark bullets remind me of Chad Pennington. His highly accurate throwing stemmed from the fact that his passes were so slow, he could run them down and correct them in mid-flight.
:laugh2:
I can see the logic for the glow-in-the-dark bullets though. If the rifle is set up to cycle subsonic suppressed and does, you don't want to accidentally stick a mag of standard ammo in it in the dark. I'm sure you would be picking up broken pieces.

The Blackout does seems to be the round to tinker with.
Rounds vary with bullet weight from 85 gr to 240 gr:eek:.
Velocities from 900 fps to 2550 fps.

I bet getting all of those to cycle :quote:good:quote: is nearly impossible.

Like getting subsonic ammo to cycle the massive bolt carrier on a RFB without changing the gas system completely:rolleyes:.
...and next thing you know, you're being diagnosed as mentally-ill:tinfoil: for following this thread: http://www.thektog.org/forum/f91/auto-gas-plug-would-possible-245940/ Text Font Line
 

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