[HK_Lova]

Why are some mags straight and some bent? For instance, MP5 9mm = bent, MP5/10 and 40 = straight; HK416 = bent but HK417 = straight...? It seems to go by caliber for rifles i.e. .308 = straight and 5.56 = bent, but proportionally they are the same. And on another note; early Waffle mags for the MP5 9mm were straight although according to HKPRO were unreliable with JHPs, why might that be? Anyone able to enlighten me?

 

[HK_Lova]

BTT; sorry guys but this one is cooking my noodle.

 

[AviatorDave]

Not entirely sure about why JHPs would cycle better in curved vs. straight mags, but I do have an idea about why some mags are curved -

If you look at the brass bullet casings, they are not perfectly cylindrical. They taper a little - smaller at the front than the rear. This helps with extraction. If they were completely cylindrical, they would be pressed against the chamber the full length as they were pulled out. With the taper, after they are pulled rearwards just a little, they get significant clearance from the chamber walls. A pistol round will need less taper than a rifle round, simply because of the shorter length. The shorter the case is, the less of it is contacting the chamber wall during extration.

So... if you lay a bunch of ammo side by side, they won't line up in a straight line. They start to form an arc. The mags are shaped to allow for this. There are a lot of factors that determine how much, if any, curve is needed - a greater taper will require more curve. A low-capacity magazine might not need to be curved as the stack of ammo may not be tall enough for the curvature to really become apparent. A single stack magaine might need more taper than a double stack of similar capacity.

 

[HK_Lova]

Good point about the mag capacity but it makes me think of the M14. The M14 chambered in .308 with a 20 round straight magazine. 20 round mags of .223 are curved even though .308 and .223 dimensions are proportionally very similar in regard to shape and contour.

 

[AKATTACK]

Good point about the mag capacity but it makes me think of the M14. The M14 chambered in .308 with a 20 round straight magazine. 20 round mags of .223 are curved even though .308 and .223 dimensions are proportionally very similar in regard to shape and contour.

The original 20 round AR15/M16 mags were straight. I guess they just never had enough feeding problems with the M14 to justify a curved mag. Probably goes the same with all the .308 weapons what had straight mags. Most of them had 20 round mags-30 rounds of .308 makes a long mag. Like I said, most of the "battle rifles" used 20 rounders and must have been "reliable enough". The higher the capacity of the "stick" mag, then the more they tend to have problems.

 

[AviatorDave]

Good point about the mag capacity but it makes me think of the M14. The M14 chambered in .308 with a 20 round straight magazine. 20 round mags of .223 are curved even though .308 and .223 dimensions are proportionally very similar in regard to shape and contour.

Actually, if you actually measure them, they are quite different. The .223 has a 20% greater taper. I just measured a few. A .223 has a .025" diameter difference over a 1.3" length, a .308 has a .020" diameter difference over a 1.425" length. That little bit might make the difference on reliable feeding vs. unreliable feeding.

I'm sure there are other factors involved like how much time is available for the mag to push the next round up into position, the relationship between the feed lips and the carrier or bolt head, etc.

Which actually gives me an idea of why the JHPs may be less reliable in the MP5 stick mags. In a stick mag, even with the slight taper of 9mm rounds, you basically will end up with the case heads all pushing tightly against each other, but some clearance or at least less pressure between the bullet ends of the casing. So in a straight magazine, the force from the spring through the follower is transmitted from casing to casing through only the back end of the bullet. And again, in a straight magazine, the front of the cartridge at the top of the magazine won't be pushed as tightly against the feed lips as the back of the cartridge. Since JHPs are shorter and lack the smooth taper that FMJ bullets do, I'm thinking that maybe since they are not pushed up against the front of the feed lips as they are pushed forwards by the bolt head, the flat nose of the bullet hits against the front of the magazine and gets jammed.

 

[HK_Lova]

I think you're on to something That all makes pretty good sense. I'd like to for certain but I think you may be onto it. Thanks