OK Pygmy, Better Have a Clean Change of Undies Handy

[wildcat junkie]

On 11/4/2016 at 9:41 PM, Trial153 said:

Nice work!

I just sold my last sporterized m98...can't say I wasn't glad to see it go.

The only parts of this rifle that are original military parts are the receiver, the bottom metal & the barrel.

 

The receiver has been milled, drilled & tapped for scope bases, the bolt handle was cut off & a new one welded on, the barrel is cut back, re-contoured & chambered in a different cartridge and the bottom metal has the locking screw holes eliminted, the trigger bow modified and the internal dimensions milled for the new cartridge.

 

It has a new stock, a 3-position Mod 70 type safety and a Timney fully adjustable trigger, To call it "sporterized" is a bit of a stretch IMO. I would think it would be closer to what would be called "custom" than sporterized.

Edited November 7, 2016 by wildcat junkie

[Trial153]

The only parts of this rifle that are original military parts are the receiver, the bottom metal & the barrel.
 
The receiver has been milled, drilled & tapped for scope bases, the bolt handle was cut off & a new one welded on, the barrel is cut back, re-contoured & chambered in a different cartridge and the bottom metal has the locking screw holes eliminted, the trigger bow modified and the internal dimensions milled for the new cartridge.
 
It has a new stock, a 3-position Mod 70 type safety and a Timney fully adjustable trigger, To call it "sporterized" is a bit of a stretch IMO. I would think it would be closer to what would be called "custom" than sporterized.

As were both mine, with the exception that replaced the barrels with Douglas barrels ....still glad to see them them go down road. Nice projects 20 years ago however not something i want at this time.

[wildcat junkie]

On 11/4/2016 at 5:47 PM, ATbuckhunter said:

Well if I go with a 7x57 cartridge it will either be the 7x57 or .257. If I go the .308 route is the work something that can be done at home or is it something that should be done by a gunsmith? 

The M98 action is what is known as a "controlled round feed" action.

 

The cartridge is in control of the bolt from the time it is stripped from the magazine until either the round or empty case is ejected. It is impossible to double feed a properly functioning M98 action. If the bolt is double stroked before the cartridge is fully stripped from the magazine, the partially fed cartridge will prevent another from being stripped from the magazine. As the original cartridge is pushed far enough to be completely stripped from the magazine, the extractor will grab the case head & "control" the cartridge.

 

In order for this function to operate correctly, the cartridges need to be held in the "stack" as precisely as possible. The internal dimensions of the magazine width are milled so that the cartridges are stacked in a perfect 60* relationship with the body of the case having contact with the magazine walls as well as the adjacent cartridges from the base to the shoulder.

 

The formula for this dimension is case diameter X cosine of 30* (.866) + the case diameter. One the vast majority of non-magnum cartridges that are based on the .473" case head diameter of the original 8 X 57 cartridge that formula for the rear width of the magazine is .473 X .866 + .473 = .883" with a few .001" tolerance to allow for variance in cartridge dimension.

 

So, the good news is that as long as you are re-chambering your M98 for .473" cartridge head diameter, the rear of the magazine is the perfect width. The bad news is that if you are going to utilize a different cartridge that has less taper than the original, the shoulders will bridge the internal width and the case heads will then "float" in the magazine.

 

Here is a drawing of an 8X57 case.

 

If we subtract the diameter of the shoulder from the diameter of the case base/head and divide it by the length from the base to the shoulder, we get a taper than equals .023" per inch.

 

Here is the 7 X 57 case.

If we use the same formula, we see that the 7X57 has slightly more taper at .025" per inch. While this is a difference, it is slight and more taper will have the case head bridging the width before the shoulder which, while not ideal, as not as bad as having the shoulders bridge.  Any "float" will be at the shoulder and will be less than what individual case tolerances would amount to. The case head, the part that the bolt must push against until the cartridge is stripped from the magazine, will be stable.

 

Since the 257 Roberts and 6mm Remington are merely necked down from the 7 X 57 case, they will work quite well in an unmodified 8 X 57 or 7 X 57 magazine.

 

The problem arrizes when a cartridge not directly based on either cartridge is employed. The 30-06 isn't that much of a problem as it has a taper that is not a drastic change form the 8 X 57 spec, but with the M48 Yugoslavian  intermediate length 98 action or its commercial counterparts, the '06 length cartridges are not a viable option.

 

So, just chamber it in a .308 based cartridge you say? Not so easy. The .308 case has far less taper than the 8 X 57 or even the '06.

 

You will notice that even though the head to shoulder length of the 7mm-08 is significantly shorter than any of the '57mm cases, the shoulder diameter is quite a bit larger. The same would be true of all .308 based cartridges.

 

If we subtract .454 from .473 and divide by 1.56, we only get a taper of .012 per inch, or about 1/2 of the '57mm cartridges.

 

The magazine width requirement at the shoulder would be .454 x .866 + .454 = .847"

At the same distance from the back of the magazine, the original cartridges would require a width of just .815", a .032" deficit.

 

While this might not seem significant, (it is) when 5 cartridges are stacked in the magazine the effect is multiplied by 2 1/2. Indeed 2 or 3 cartridges in the magazine would not seem to affect the feeding that much, but when 4 of 5 are loaded, getting those 1st few cartridges to feed reliably can become a crap shoot.

 

So, back to the question as to whether this requires a "gunsmith" of can be done at home. While it doesn't require a "gunsmith" persee, it isn't something one can do at home unless one has access to a mill. It can be done with a file, but that would extend to the bottom edges of the box which would be unsightly when the floorplate was released and it would significantly weaken the box. This operation need to at least be farmed out to a local machinist with a milling machine that understands the required operation.

Which begs the question; Why bother? If you don't reload, it will be easier to find .308/7mm-08/243 Winchester ammunition. However, if you do reload, the 8X57 will push a 200gr bullet at  higher Mv as a 180gr bullet from a .308 when loaded to similar pressure. A 7 X 57 will out preform the 7mm-08 by as much as 150 fps when similar bullets are loaded to similar pressure. The same can be said of the 6mm Remington compared to the .243.

The intermediate action 98s will not saddle one with a "short action" 2.80" cartridge length. They will allow up to a 3.20" OA cartridge length so why saddle it with a short stubby cartridge?

I am building a 7 X 57 on a VZ500 intermediate length M98. The barrel will allow a COAL of about 3.150" with the bullet "just off the lands" once the long chamber is head-spaced in my action.

 

Edited November 7, 2016 by wildcat junkie

[wildcat junkie]