Droog's quest for a long-barrel AK got me thinking.
... inspired me to further research AK barrel blanks. By "AK Barrel Blank" I mean a barrel that is "full diameter" and is capable of conversion into any standard barrel configuration.
Here's what I've dug up... in short, .303 British barrels can be converted for use as an AK barrel, in all flavors that use standard .311 bullets.
NOTE: A prime application could be replacements for Yugo M70 "shot out" barrels!
.303 British specifications:
bullet diameter .311-.312",
maximum COL 3.075",
maximum case length 2.222",
SAAMI MAP 45,000 cup.
Twist: 1 in 10"
bullet diameter .308" or .312" (depending on rifle),
maximum COL 3.048",
maximum case length 2.115".
Twist: 1 in 9.5" RH
bullet diameter .311",
maximum COL 2.20",
maximum case length 1.528",
MAP 50,000 cup.
Twist: 1 in 10"
.308 caliber is too small for use in a .311 barrel. Of interest, "standard" .30 caliber cartridges are the same diameter- .30-06 is a .308 diameter. Therefore, a heavy-duty .308 barrel could be adapted from 1919 .30-06 barrels... food for thought.
Normally we're talking about 7.62x39 and that is what I'll limit this to for now.
The 7.62x39 cartridge dimensions are slightly smaller than a .303 Brit cartridge. This means that a .303 Brit chamber can be converted to 7.62x39 by trimming the breech just under .7" to make up the difference. Once an extractor notch is cut, the barrel journal sections could be spun on a lathe. The cartridge specs make it appear that the chamber would not need to be reamed (or for that matter, sleeved) to work with the 7.62x39 cartridge.
There is an "Enfield K" project out there that converts Enfield barrels to 7.62x39 for a bolt-action conversion. Enfield Mk 4 barrels are reportedly "thick enough" for use with AK barrel attachments. Due to the "loose fit" of the smaller 7.62x39 cartridge, a "shot out" barrel with enough throat erosion could cause tumbling of the 7.62x39 bullet, therefore requiring chamber reaming to compensate for the wear. Worn riflings would not be fixable.
Here is the link to the "Enfield K" project:
Northridge sells Enfield Mk4 barrels for $50 or so. I think these are 27" but am not sure.
Bren barrels are 25" long but have a gas port cut into them. I do not know the location, but I will assume they are not in the proper location for an AK gas port. I am not certain how easy it would be to plug-weld or "pin" this hole closed. I have inserted a threaded pin to close off a 45-degree hole on my AK-103 project, so I know it can be done. The uncertainty is that I am not sure how the threaded plug could be secured in a Bren barrel. I have been told from some "in the know" welder folks that MIG/TIG welding will induce enough heat to kink the barrel. I have also been told by "other" welder folks that this is negligible and can be safely welded. This may or may not affect accuracy, and I have no data to confirm/deny this. IF this is acceptable, this is a prime source for replacement RPK barrels!
Now about 7.62x54R:
ROMAK and Dragunov (and PKM) barrels: The 7.62x54R cartridge is slightly larger diameter which means a chamber reamer would be necessary, but the bullet would work in a .303 barrel. Since there are enough Mosin-Nagant (and Mauser) barrels out there, this may represent a barrel resource that could allow us to build Dragunov-style rifles. Since the ROMAK-3 is for all practical purposes a long-barrel AK, this has obvious applications with using only a standard AK kit.
PKM barrels would most certainly need to be as thick as an RPK build in order to ensure a safety margin, and Bren barrels could probably be fit to this purpose. I don't have a PKM kit - yet - but I'd like to have a spare barrel for it.
Safety concerns should be negligible, since the chamber pressures are comparable. Safety of a 50+ year old barrel may be a concern due to metal fatigue, but I am not certain if this can be measured or quantified in any way. It may be of no consequence at all.
In summary, Enfield and Bren barrels share enough dimensions that they could physically be converted to use the AK cartridge. A lathe is required, a mill is a good idea. A chamber reamer is required for 7.62x54R caliber.
I'm open to all comments about recycling 303 barrels in this manner...