RPK vs. AK: velocity and range

[Caliburn]

I was looking at the info comparing AKM and RPKs on this site:
http://www.sovietarmy.com/small_arms/rpk.html
http://www.sovietarmy.com/small_arms/akm.html

It said the difference in barrel length is 8" but the difference is muzzle velocity is only 50 fps. Huh? Does the x39 cartridge not have the horsepower to take advantage of the longer barrel? For .308 or 5.56 people say each inch of bbl equals 50 fps so the difference between AK/RPK should be huge.

Also those sites listed the effective range of the AK as 300 meters but the RPK as 600. How can that be, without additional velocity?

Has anyone chono'ed the difference?

Also, how is the accuracy of the RPK? Does the longer barrel/ sight radius really tighten up the groups? Maybe that makes the difference?

Thanks for any advice!

 

[DMK]

The 7.62x39 was designed for shorter barrels. In the long RPK barrel, most of the powder is burned up before the bullet leaves the muzzle and drag starts to become a factor.

 

[benEzra]

For .308 or 5.56 people say each inch of bbl equals 50 fps so the difference between AK/RPK should be huge.

Higher-velocity rounds tend to be more sensitive to barrel length than lower-velocity rounds, I think. (Nominal velocity of 7.62x39 is 2350 fps.) Also, the 7.62x39 was originally designed for the roughly 16" barrel of the AK-47, whereas the original M16/AR-15 (which the 5.56 NATO was designed for) had a 20" barrel. So the 7.62x39 burns most of its powder in 16 inches, but the .223 still has powder left to burn at that length.

Also those sites listed the effective range of the AK as 300 meters but the RPK as 600. How can that be, without additional velocity?

A light machine gun/squad auto generally is considered to have a greater effective range than a regular rifle, due to the greater number of rounds on target. I know the cyclic rate is effectively the same, but the RPK is designed for sustained, accurate full auto. Just like the M249 is probably ascribed a greater effective range than the M16 (is it?)

 

[Caliburn]

Thank, Ben, that makes sense. But

Also, the 7.62x39 was originally designed for the roughly 16" barrel of the AK-47

Are you sure about that?

The x39 cartridge was designed in 1943, before the AK, and was first used by the SKS with its 20" bbl. The SKS saw service in WW2 in '45 but the AK came afterwards (and then saw service everywhere).

All that would lead me to believe that the 24" RPK barrel must do something good for it, or they would have chopped it shorter to save weight.

Right?

 

[Nightcrawler]

Sight radius is a factor too, as is the bipod.

The SAW actually has a shorter barrel than the M16 (16"), yet the battle sights elevate out to 1200 meters (I think...the M16A2s go out to 800, which is really, really stretching it for 5.56mm...).

Mostly, though, the cyclic rate increases the effective range of machine guns. You're just more likely to hit a target at distance with a stable, full-auto platform. (SAW can really rattle you when firing though...short bursts only, otherwise you lose your sight picture.)

 

[benEzra]

The x39 cartridge was designed in 1943, before the AK, and was first used by the SKS with its 20" bbl. The SKS saw service in WW2 in '45 but the AK came afterwards (and then saw service everywhere).

All that would lead me to believe that the 24" RPK barrel must do something good for it, or they would have chopped it shorter to save weight.

Good point, I hadn't thought about that. I just noticed that when firing my 16" barreled SAR-1, I get a LOT less muzzle flash than my mini-14, even though the mini has an 18.5" barrel. I wonder if most x39 is set up with a fairly fast burn rate to get more oomph out of shorter barrels. As far as why the 24" barrel on the RPK, sight radius, reduced muzzle blast, and less rotational moment during recoil would be possible reasons; it would make it less portable but more stable during automatic fire.

For you guys with .300 Whisper AR uppers, how sensitive is this round to changes in barrel length beyond 16" (with full-power loads, not subsonics)? Ballistically, it looks almost exactly like a 7.62x39 from what I've seen, so I wonder if there's any sort of trend in the short .30's.

 

[10shooter]

The .223 is a high velocity varmit round its bore diameter to case capacity is much higher than the 7.62x39. .223 needs and wants longer barrels to get the most out of its performance. The 7.62x39 just doesn't need much barrel relatively light bullet compared to its bore size low pressure 40,000 psi small powder charge. Don't let the sights fool you stocked Browning 9mms have 800 yard sights.

 

[Caliburn]

Thanks, guys. That all makes sense.

By the way - the High Road ROCKS! AK-47.net has a special RPK forum. I posted this question there before here and no one there replied.

High Roaders rule.