[judcargile]

Let is assume that I wanted to take 6-8" off of the unit to make it super useful in a vehicle or just because it would be something to tinker with, if nothing else.
I suppose the gas would be a bit stronger as it would build inside of less barrel space.
I would most likely end up adding weight to the shorter bolt carrier and certainly start with a fresh unit, because it may be beyond my ability. I would be interested in your thoughts other than how impractical it may seem. I have seen ARs with 7" inch barrels perform well @ 100 yards; So, why not something with more azz than a krinkov?

 

[MadDogDan]

My experience with SBR's tells me the opposite. I have 14 SBR's with barrel lengths of 5", 7.5", 10", 10.5", 11", & 12" and all of them had weaker gas pressure when shortening the barrels. Gas blocks are originally positioned so that the requisite amount of pressure is bled off before most of the gas escapes the muzzle. When you shorten the barrel too much of the gas exits the muzzle before the required amount of gas needed to cycle the action can pass through the port. That is why in most cases you need to add a muzzle device that increases pressure like a Noveske Flaming Pig or a PWS CQB. If you don't use one of those devices you need to open up the gas port. Ideally you want a minimum of 2" of barrel past the gas port. I cut down the barrel on one of my AUG's to 13" but I needed to put a Flaming Pig on the end to get it to work properly.

My 18" RFB has 6" of barrel past the gas port so I imagine you cou could chop it down to 16" and it would still perform without modification. If it was under gassed adding the gas rings to your piston would probably take care of it.

No modification would be needed to the carrier just the ability to regulate gas pressure to the proper levels for reliable functioning.

 

[Junkie]

Impulse delivered to the BCG depends on port size, port pressure, and dwell time.

Port size is based on what hole you drill, port pressure is based on how far from the breech the port is (and ammo type), dwell time is based on how barrel there is past the port (and how far out the port is, and ammo type). Dwell time isn't measured just in inches: on an AR with a pistol length gas system you might get away with 2" of barrel beyond the port (I'm making up this number to display the concept, my numbers may be wrong), because the port pressure is relatively high and the bullet is moving relatively slow, but with a rifle length gas system you need more barrel beyond the port due to a lower gas pressure and higher bullet velocity.

If you're building a whole new barrel to relocate the gas block, and probably a whole new forend to accommodate that, and so on, you can probably manage to shorten it significantly. But... I'd rather have enough room for my hands, and enough barrel length to get a decent velocity - I don't think 308 is a good choice on short barrels for anything but a toy.

I agree that you could likely get away with cutting it to 16". If you are shortening it, I'd do it a little at a time and then check required gas system settings to see if you can likely get away with shortening it more. I'd also make sure that Keltec is willing to sell you a new barrel, or have a gunsmith who knows what they're doing make you a custom barrel (which will NOT be cheap).

 

[TNC]

I fully support anyone's desire to pursue any kind of project for any personal reason. Stuff like that can be fun. From my perspective, however, I'm not seeing the logic in an SBR RFB. You already have a bullpup length firearm, so the length issue has been pretty much solved. I might understand it a little more if it was a 5.56 gun, but a .308 doesn't seem to fit that category as well IMO. The other reason is that the RFB is already a challenge for many to balance the gas system to particular ammo and/or suppressors, so it seems like you'd be opening up a bigger can-o-worms.

Again, this isn't a slam to the pursuit of the concept, but just a 2-cents observation...worth probably less than 2-cents...LOL!

 

[MadDogDan]

I fully support anyone's desire to pursue any kind of project for any personal reason. Stuff like that can be fun. From my perspective, however, I'm not seeing the logic in an SBR RFB. You already have a bullpup length firearm, so the length issue has been pretty much solved. I might understand it a little more if it was a 5.56 gun, but a .308 doesn't seem to fit that category as well IMO. The other reason is that the RFB is already a challenge for many to balance the gas system to particular ammo and/or suppressors, so it seems like you'd be opening up a bigger can-o-worms.

Again, this isn't a slam to the pursuit of the concept, but just a 2-cents observation...worth probably less than 2-cents...LOL!

For some of us the only reason to SBR something is "because we can". I have something like 14 SBR's and some of them could definitely ballistically benefited from a longer barrel. However, they just wouldn't be as cool. When was the last time you saw a 16" or 18" Beowulf? I rarely see them over 9" and I am sure they loose a lot ballistically being shortened.

I like my RFB at 18" but I definitely can see the appeal of a "shorty" RFB. There is something really cool about seeing the muzzle terminate at the end of the handguard.

 

[850sub]

I sure wouldn't want to shoot an SBR RFB more than a couple times. Unless it was gonna have a dedicated suppressor. Wow that this loud.

 

[BlakeHanson]

.. That is why in most cases you need to add a muzzle device that increases pressure like a Noveske Flaming Pig or a PWS CQB. If you don't use one of those devices you need to open up the gas port. Ideally you want a minimum of 2" of barrel past the gas port. I cut down the barrel on one of my AUG's to 13" but I needed to put a Flaming Pig on the end to get it to work properly..

I sure wouldn't want to shoot an SBR RFB more than a couple times. Unless it was gonna have a dedicated suppressor. Wow that this loud.

You hit on an interesting concept

Since almost everyone that puts a suppressor on their RFB has to deal with the resulting over-gassed problem, shortening the barrel would cancel it out.
You could have a suppressed RFB carbine that was practically the same length as the original and if the right "recipe" was cooked up, no modifications to the gas system would be needed at all.
It would be a dedicated suppressed gun but most guys running a suppressor never take them off (why:nuts so no big deal.
Granted it wouldn't be a 'stubby' SBR but still cool.

On the other side, I don't know what it would take to raise the eyebrow of the government, but a SBR in .308 with a :quote:silencer:quote: on it might just do it.

 

[MadDogDan]

I sure wouldn't want to shoot an SBR RFB more than a couple times. Unless it was gonna have a dedicated suppressor. Wow that this loud.

I imagine it would be a lot like my 7.5" SBR'd AK (7.62x39). I shoot that sometimes in 3-gun matches but you have to shoot with your mouth open to mitigate some of the concussion you feel in your brain.

 

[Junkie]

For some of us the only reason to SBR something is "because we can". I have something like 14 SBR's and some of them could definitely ballistically benefited from a longer barrel. However, they just wouldn't be as cool. When was the last time you saw a 16" or 18" Beowulf? I rarely see them over 9" and I am sure they loose a lot ballistically being shortened.

I like my RFB at 18" but I definitely can see the appeal of a "shorty" RFB. There is something really cool about seeing the muzzle terminate at the end of the handguard.

Thumper cartridges don't gain all that much from long barrels.

To benefit from a long barrel, you need to have high gas pressure at that point. You get higher gas pressure from two things: larger case volume and smaller bore size. With a .22 bore, you don't need as much gas to fill the barrel as a .50 bore, so for the same case volume you have higher pressure at 12" (or whatever distance you're looking at) with a .22 vs .50

Obviously .50 Beowulf has more case volume than 5.56, but not by as much as the difference in barrel volume. Sadly, I'm having trouble finding the volume... so lets look at 458 SOCOM, at 61.6gr water (it's the most similar I could find info for). In comparison, 5.56 is 28.5gr water.

5.56 bullet diameter is .224, 458 SOCOM bullet diameter is .458. 5.56 bore cross section is .0394, 458 is .1647. Case volume / bore area (using odd units, but I'll use the same for both) is (28.5/.0394)=723 for 5.56, (61.6/.1647)=374 for 458. This means that, all else equal, you'll have significantly less advantage from a longer barrel with 458 vs 5.56, similar to how 300BLK doesn't gain much (I'm seeing around 25gr water, bullet diameter .308, bore area .0745, volume/area 336, very similar to 458).

I'm not denying that you gain some velocity from a longer barrel with 458, but it's generally much less important vs 5.56 or other small bore rounds. If you look at small bore magnum rounds, barrel length is even more important.