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No. 4 Conversion to 7.62
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britrifles View Drop Down
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    Posted: December 07 2022 at 5:25pm
A number of members have asked about conversions of the No. 4 .303 to fire the 7.62 NATO cartridge.  

Early trials in Canada by the DCRA were done by modifying a .303 barrel to chamber the 7.62 with good results.  The following photos are of a January 1961 Canadian Marksman article written by Dave Reynolds of the DCRA that was in my Dad’s files.  Hopefully they are clear enough to read. 

Unfortunately, I’m not smart enough to know how to stitch these together as a single PDF file.  I’ll try to enlist the help of a more technically savvy member to do that for me. 
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That is a bad way to convert, to me anyways. 7.62 bore is .30, .303 is way too big. Where are you going to find a .303 barrel that is .301 to .302? Even .302 is getting too big for 7.62 as I like under .302 for my .30 cal rifles. Not a good way to do this IMHO.

Thanks for the document! Interesting.
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britrifles View Drop Down
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.301 to .302 is quite common.  And it worked quite well even with a .303 bore, read Para 4.  This was done for DCRA SR(b) Matches with good results.

It’s also been shown that .308 bullets will shoot quite good when loaded into the .303 case.  So, not surprising it works quite well when re-chambering a .303 barrel to 7.62.

Agree, a better way is to find a 7.62 barrel for the No. 4, but good luck finding one. 
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Originally posted by britrifles britrifles wrote:

Agree, a better way is to find a 7.62 barrel for the No. 4, but good luck finding one. 

Yup, agreed. It just does not sit well with me. I'm usually a stickler for tight bores. Starting out with a .302 bore for a .30 cal round is just backwards to me. It may have worked well enough, but I don't like it. Lol!

All of my U.S. Military .30 cal rifles are well under .302, most under .301
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scottz63. Just a FYI. An actual 7.62 projectile is .309" (Source 7.62x39) and as britrifles mentioned, it is not uncommon for the Enfield to have bore measurements of .301" - .302". My BSA 5 groove measures @ .300". An old target shooter buddy of mine got me invested into shooting 168grn .308" BTHP's out of a No4Mk1* with a 2 groove barrel because it will grip the bullets a tad better than the 5 groove. I have even used the Lapua 200grn 7.62x54R .3105" FMJ Rebated Boattail.  In retrospect of what britrifles has shown to us, doing the necessary work and rechambering a .303B to 7.62x51mm is fairly straightforward and will still offer good accuracy results using projectiles between 150 and 180grn with better than average results using the 155grn and the 168grn match grade projectiles. DCRA actually did this procedure to many No4Mk1/Mk2 rifles with no issues...
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I get it, and understand that it works. Just sounds odd to me. 
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scottz63 View Drop Down
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Just checked my No4 sporterized barrel. I has the best bore out of all 3 of my No4 barrels. Too bad it has been cut down.
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britrifles View Drop Down
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Hard to read that, is it .3020? Or .3015? 
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.302
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britrifles View Drop Down
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Originally posted by scottz63 scottz63 wrote:

I get it, and understand that it works. Just sounds odd to me. 

Yes it does.  But there was a pretty compelling reason to do this.  The DCRA matches relied on government issue ammunition at the match.  That was to help keep cost to the shooters down and have a level playing field.   The stocks of .303 ammunition were on the decline by 1960 and new 7.62 No. 4 barrels were not yet available.  
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Got it. :)
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Is the mating of Ishapore 2a .762x51 barrels to No4 actions I’ll advised or unworkable?
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Goosic has successfully done this. He should be along here to comment.  
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Originally posted by Moosm14 Moosm14 wrote:

Is the mating of Ishapore 2a .762x51 barrels to No4 actions ill-advised or unworkable?
Definately not unworkable just really time consuming,  and only ill-advised if you do not have the proper skill set to do it correctly.

 There is alot of fitting up and re-fitting up just to get the barrel indexed correctly which involves removing excess metal from the barrel that contacts the reciever. Once that is accomplished, the next step is to check the headspace using 7.62 headspace gauges and not 308W. This is the time consuming part. You will have to shorten the boltface by stoning it until the correct headspace is achieved and that take alot of patience.  A 2A1 magazine can be modified to fit into the No4 mag well but keep in mind that cartridge extraction is going to be hit or miss.
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Thank you very much for posting this britrifles.

Moosm14, to add to Goosic's comment, it's actually very easy to re-index the 2A1 barrel if you have access to a lathe. The shoulder needs to be shortened by about 35 thousandths. It's pure pain if you only have hand tools... But trivial if you have a lathe. You may need to file the extractor cut a bit deeper to match the deeper chamber.

The headspacing part is also very easy if you have a chamber reamer. I've never finish-reamed/set headspace on a lathe, but you can readily rent a pull-through reamer (at least in the US) from reamerrentals.com or others. You cut the chamber deeper rather than grinding the bolt face down and re-heat-treating it.

A .308 Win headspace gauge should be fine. It's marginally shorter than a 7.62 NATO gauge. The 7.62 NATO leaves looser tolerances in the chamber to maximize reliability in military applications. Headspacing to .308 Win spec should leave a tighter chamber and improve potential accuracy.

Where you need to be careful is in the throat. The 7.62 NATO has a longer throat, which decreases chamber pressure vs. the .308 Win. If you use a .308 Win reamer, make sure you load very mild loads. If you use the 7.62 NATO reamer, you should be fine, but don't seat your bullets way out, or your throat will effectively be the same as if you'd reamed to .308 Win. specs.

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Originally posted by lawndart lawndart wrote:

Thank you very much for posting this britrifles.

Moosm14, to add to Goosic's comment, it's actually very easy to re-index the 2A1 barrel if you have access to a lathe. The shoulder needs to be shortened by about 35 thousandths. It's pure pain if you only have hand tools... But trivial if you have a lathe. You may need to file the extractor cut a bit deeper to match the deeper chamber.

The headspacing part is also very easy if you have a chamber reamer. I've never finish-reamed/set headspace on a lathe, but you can readily rent a pull-through reamer (at least in the US) from reamerrentals.com or others. You cut the chamber deeper rather than grinding the bolt face down and re-heat-treating it.

A .308 Win headspace gauge should be fine. It's marginally shorter than a 7.62 NATO gauge. The 7.62 NATO leaves looser tolerances in the chamber to maximize reliability in military applications. Headspacing to .308 Win spec should leave a tighter chamber and improve potential accuracy.

Where you need to be careful is in the throat. The 7.62 NATO has a longer throat, which decreases chamber pressure vs. the .308 Win. If you use a .308 Win reamer, make sure you load very mild loads. If you use the 7.62 NATO reamer, you should be fine, but don't seat your bullets way out, or your throat will effectively be the same as if you'd reamed to .308 Win. specs.

Lawndart
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...
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