Just thought I'd share my experience with my MAS-49/56. I picked it up about a year ago at Carolina Gunrunners for a reasonable price. It is one of the CAI conversions to 308/7.62, which I had heard some horror stories about. But being the closet hipster that I am, I decided that an AR-10 or M1A was "too mainstream," and way too expensive to be used as a range toy at any rate. And as an added plus, until I had personally fired it I could use the tired old joke that, being a French rifle, it had never been fired, and only been dropped once.
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It seems the Century didn't to a fantastic job on these conversions. From what I have gathered, they removed the barrel and chamber, cut about 3mm off the chamber end, and re-reamed the chamber to accommodate a 308. They also apparently snipped 2 coils off the end of the recoil spring to try prevent the heavy free-floating firing pin from causing slamfires on soft commercial primers (which I have experienced with this gun, in spite of this "fix").
Steel cased ammo seemed to run okay, but I was getting stuck cases, FTEs, FTFs, etc. on brass cases. I deduced that these problems were caused by four separate factors which were conspiring to cause problems.
1) The original 7.5x54 French is a much lower-pressure cartridge than either .308 Win or 7.62x51 (40k PSI vs 52k - 60k PSI). This means that gas entering the gas tube is moving faster than originally intended, which is affecting the timing and bolt thrust, initiating extraction before the casing has started to contract after expansion.
2) By shortening the barrel and gas tube by 3mm, the geometry of the gas system is changed, which may be affecting the timing and bolt thrust, initiating extraction before the casing has started to contract after expansion.
3) By shortening the recoil spring, the bolt is resisting rearward movement less than originally intended, which is probably affecting the timing and bolt thrust, initiating extraction before the casing has started to contract after expansion.
4) The chamber reamer they used evidently wasn't the sharpest tool in the drawer, because the chamber was noticeably rough, which is causing more resistance in the extraction process which is already accelerated by the first three factors.
All of which would explain why steel ran okay (doesn't expand or contract as much) where brass gave me fits. After the first few range trips with it, I shrugged, thought "you get what you pay for," and contented myself at having a neat historical piece that isn't a fantastic shooter. I had read others' accounts with these conversions, and saw that some had had success installing a variable gas system, but that sounds expensive, and who knows is the guy who was making them 6 or 8 years ago is still making them.
Recently I started handloading to try to insulate myself from ammo supply issues. While I was buying dies, I found a set of Lee RGB dies for .308 in stock, and thought "why not?!" And then it occurred to me: perhaps I can mitigate some of these issues by loading .308 down to resemble the 7.5 French this gun was designed for. I figured there wasn't much I could do about #2 and #3 on the list. I addressed #4 by mounting a spent 308 casing on a bolt and using some polishing compound to gently lap the inside of the chamber to smooth it out a shade.
The only rifle powder I have on hand is IMR 4320. IMR doesn't seem to have any data for 7.5 French, but they do for the similarly-sized, similar-pressure .30-40 Krag, and for the similar 7.5 Swiss. So by comparing the Krag and Swiss and reading between the lines with the given loads and pressures for .308 Win, after some interpolating and calculating I came up with a guesstimate of 39 gr. over a 147 gr. surplus FMJBT. I loaded up a box of 20 just to see how it would work.
Went to the range to try it out, and got off all 20 rounds with only a single magazine-related feeding problem; no extraction issues.
It seems the Century didn't to a fantastic job on these conversions. From what I have gathered, they removed the barrel and chamber, cut about 3mm off the chamber end, and re-reamed the chamber to accommodate a 308. They also apparently snipped 2 coils off the end of the recoil spring to try prevent the heavy free-floating firing pin from causing slamfires on soft commercial primers (which I have experienced with this gun, in spite of this "fix").
Steel cased ammo seemed to run okay, but I was getting stuck cases, FTEs, FTFs, etc. on brass cases. I deduced that these problems were caused by four separate factors which were conspiring to cause problems.
1) The original 7.5x54 French is a much lower-pressure cartridge than either .308 Win or 7.62x51 (40k PSI vs 52k - 60k PSI). This means that gas entering the gas tube is moving faster than originally intended, which is affecting the timing and bolt thrust, initiating extraction before the casing has started to contract after expansion.
2) By shortening the barrel and gas tube by 3mm, the geometry of the gas system is changed, which may be affecting the timing and bolt thrust, initiating extraction before the casing has started to contract after expansion.
3) By shortening the recoil spring, the bolt is resisting rearward movement less than originally intended, which is probably affecting the timing and bolt thrust, initiating extraction before the casing has started to contract after expansion.
4) The chamber reamer they used evidently wasn't the sharpest tool in the drawer, because the chamber was noticeably rough, which is causing more resistance in the extraction process which is already accelerated by the first three factors.
All of which would explain why steel ran okay (doesn't expand or contract as much) where brass gave me fits. After the first few range trips with it, I shrugged, thought "you get what you pay for," and contented myself at having a neat historical piece that isn't a fantastic shooter. I had read others' accounts with these conversions, and saw that some had had success installing a variable gas system, but that sounds expensive, and who knows is the guy who was making them 6 or 8 years ago is still making them.
Recently I started handloading to try to insulate myself from ammo supply issues. While I was buying dies, I found a set of Lee RGB dies for .308 in stock, and thought "why not?!" And then it occurred to me: perhaps I can mitigate some of these issues by loading .308 down to resemble the 7.5 French this gun was designed for. I figured there wasn't much I could do about #2 and #3 on the list. I addressed #4 by mounting a spent 308 casing on a bolt and using some polishing compound to gently lap the inside of the chamber to smooth it out a shade.
The only rifle powder I have on hand is IMR 4320. IMR doesn't seem to have any data for 7.5 French, but they do for the similarly-sized, similar-pressure .30-40 Krag, and for the similar 7.5 Swiss. So by comparing the Krag and Swiss and reading between the lines with the given loads and pressures for .308 Win, after some interpolating and calculating I came up with a guesstimate of 39 gr. over a 147 gr. surplus FMJBT. I loaded up a box of 20 just to see how it would work.
Went to the range to try it out, and got off all 20 rounds with only a single magazine-related feeding problem; no extraction issues.
