Gunsmithing Internal magazine length vs. COAL question

[BurnOut]

So, as a rule of thumb, does anyone have any guidelines regarding how much clearance should exist between the internal walls of a magazine and a loaded cartridge in order to prohibit good feeding? 0.100"? 0.200"?

Let's say that I have an internal magazine length of 2.500" (just an example for the purpose of this discussion), what would be the max COAL that I would most likely be able to reliably feed from that magazine? I understand that there are a number of variables that could impact magazine feeding (shape of the feed lips, mag spring pressure, etc...), but I am looking for just kind of a rough guideline to shoot for.

TIA

 

[Wannashootit]

http://precisionrifleblog.com/2012/12/13/detachable-magazine-dimensions-max-coal/

 

[BurnOut]

http://precisionrifleblog.com/2012/12/13/detachable-magazine-dimensions-max-coal/

I saw that, but I'm not seeing any comparison between the recommended COAL and the actual internal magazine length measurement.

 

[Wannashootit]

Sorry- misread that you're asking about an internal mag. Don't know of a hard and fast rule- I try not to have less than .025-.030- but plead guilty to less occasionally..

 

[BurnOut]

FWIW, I just measured an AW/AT mag internal length at about 2.983, so based on the info at PRB indicating that max recommended COAL is 2.880, it looks like the answer that I'm looking for is .100" difference between the COAL and the inside of the mag is a good rule of thumb.

 

[Forward543]

I second the .025-.030". Seems to be pretty reliable.

 

[Supersubes]

I think it's too combination specific, and you noted. Recoil, case taper, magazine design, they all play a role. The answer would be something like "short enough to not impede the function" Duh right? My axmc 338 cannot be loaded to mag length at all, because the top round rides forward during recoil. Once the bolt is opened, the meplat for that top round will get caught under the feed ramp (dumbest thing ever, cant believe they designed it that way). Also, the plastic tipped bullets like eld's get totally smashed during recoil by the front of the magazine. AI mags have a rib that are supposed to engage the case shoulder and prohibit forward movement, and they do, but there's some wiggle room there. So on that gun, .1" is about right.

I shoot a lot of hornady eld's, and dont like the nice polymer meplats getting bashed, so thats my guide. I load ten in my AW mags, shoot nine of them, and then check the meplat of the tenth. Thats for rounds like the .260. The creedmoor and x47 are nowhere near the end of the mag, so I ignore it.

 

[BurnOut]

I think it's too combination specific, and you noted. Recoil, case taper, magazine design, they all play a role. The answer would be something like "short enough to not impede the function" Duh right? My axmc 338 cannot be loaded to mag length at all, because the top round rides forward during recoil. Once the bolt is opened, the meplat for that top round will get caught under the feed ramp (dumbest thing ever, cant believe they designed it that way). Also, the plastic tipped bullets like eld's get totally smashed during recoil by the front of the magazine. AI mags have a rib that are supposed to engage the case shoulder and prohibit forward movement, and they do, but there's some wiggle room there. So on that gun, .1" is about right.

I shoot a lot of hornady eld's, and dont like the nice polymer meplats getting bashed, so thats my guide. I load ten in my AW mags, shoot nine of them, and then check the meplat of the tenth. Thats for rounds like the .260. The creedmoor and x47 are nowhere near the end of the mag, so I ignore it.

That's fair, and I appreciate the response. In my case, I have an internal box mag with a hinged floor plate (BDL-style) and a plastic liner, so I'm hoping to avoid plastic meplat deformation like you're describing. Mine will be stout-ish in terms of recoil (the plan is for a 26" CF barrel and .338 WM running 250 gr projectiles as close to 3k FPS as I can get them), so I absolutely expect some movement of loaded rounds in the mag.

 

[Supersubes]

I just built myself a 338-06 using a saami reamer. I jump the barnes bullets about .150. COAL is 3.3", and they still whack the front of the box which is 3.6". Maybe the run their getting at the front of the box is whats flattening out the tips? Im not mashing the barnes very bad or going for long shots, so its no issue for me.