No. 4 Conversion to 7.62

Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic

 Goosic wrote:

You actually should never use 308 headspace gauges for a rifle chambered in 7.62x51mm.  There are specific reasons as to why this is ill-advised. Another point that needs to be addressed here is that you need to have whatever bolthead you will be using re-proofed to that particular calibers maximum chamber pressures. If not done accordingly, the potential for a catastrophic bolt failure is highly plausible.  When I did my conversions, I followed a very specific set of instructions. This is NOT a "Plug-N-Play" situation lawndart and headspacing is the most critical part of the conversion step. Use the correct NATO gauges ONLY! There is more to just cutting the shoulder back on the 2A1 barrel to get it to index correctly that many a first-timer gunsmith overlooks as well lawndart...

That "first-timer gunsmith" jab rubbed me the wrong way. By all mean, please tell me about those "specific reasons".

I may not know much about the Enfield, but I'm a mechanical engineer and an 07 FFL. I also spent a good amount of time working in my unit's armory while in the Army. I'm always eager to learn, but am certainly not a "first-timer gunsmith". I've headspaced dozens of rifles, and checked hundreds.

The difference in standard Go gauge dimensions between 7.62 NATO and .308 Win is 0.0055", or 5.5 thousandths of an inch. The armorer's Go gauge used for the L42 is actually 2 thousandths shorter than the .308 Win. The one for an FAL is a mere 2.5 thousandths longer. For reference, these variances are roughly what you'd get if you rotated your FL sizing die by 1/32nd of a turn.

The differences relate to fits, tolerances, and reliability, not pressures or safety. Yes, you may have a bit more trouble closing your bolt on a dirty, corroded, de-linked Indonesian 7.62 NATO cartridge if you chambered your rifle using a .308 Win Go gauge... But as long as you don't need the ability to blast bottom-shelf military ammo with 100% reliability, .308 Win headspace spec will give you a tighter chamber and higher accuracy potential. That's why the L42 is chambered to an even shorter headspace spec than .308 Win.

Keep in mind that I'm not talking about the throat. As I mentioned earlier, you need a 7.62 NATO chamber reamer with a properly-dimensioned throat to avoid running into pressure issues caused by the shorter throat/leade of the .308 Win.

Also keep in mind that I'm not talking about No-Go or Field gauges. Essentially, .308 Win No-Go and Field gauges tell you nothing when inserted into a 7.62 NATO chamber. But it's important to note that these gauges are simply QC checks in the context of finish-reaming a chamber. You'd be very hard-pressed to ream past a No-Go gauge if you're working carefully and checking often.

Now back to the subject of Enfields, where I don't know very much...

Were the bolt heads really any different, or were they simply re-proofed as part of the QC process? It would be good to know what proportion of them failed the re-proof, but I'd be very surprised if the vast majority of non-military conversions out there were not re-proofed to military standards. I expect that would only be a factor if you've taken a significant amount of material off your bolt face to set headspace rather than reaming your chamber deeper.

Lawndart