Posted: 2/12/2008 1:36:33 PM EDT
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I own several P-mags and have always been pleased with their durability and performance. Then one day I left my AR and mags in the trunk of my car overnight (a cold night, around 0 degrees Fahrenheit) and went shooting the next morning (around 20 degrees Fahrenheit out) The Pmags were very difficult to insert into the lower receiver for some reason, I tried an equally cold aluminum milspec mag and it clicked right in like always. I was a little embarrassed because I was going to show a buddy how great the P-mags were.He I tried the P-mags again the next day when they were at room temperature and they clicked right in the lower again with no problems. I still like the P-mags, but perhaps if the mags get cold, I will rely more on aluminum or steel mags. I figure that the polymer mags either expanded in the cold or contracted enough in one part to bow out enough to be out of spec in another. Is this an uncommon or known issue? |
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I agree, Sometime in the multitude of all the chemistry classes I took, I remember that things get smaller when they get colder. And, at different rates based on the properties of the material! The plastics used in the Pmags probably didn't shrink very much, the plastic seems very dense so that would make sense. Your magwell and your aluminum mag are made of a very similar material so shrunk the same amount. Thus, the tolerances were the same as usual! Aluminum and most metals usually shrink or expand (under heat) quite a bit because there is lots of room and wiggle ability between the atoms for them to do so. Not very much magpul can do about the laws of physics. 
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We have a couple of guys in Alaska who have tested the PMag at -30 with no issues including seating in the mag well. One has a video of him shooting PMags vs USGI ALU. PMags ran without a hitch. |
He wasn't very impressed.
I would assume the Pmags would have been well tested in a temperature extremes already, that is what made me wonder what was going on.. I thought about what cold would do to the polymer. Fluids expand and solids contract. Then I got to thinking if hard polymers would fall into the non-Newtonian fluid category and just confused myself thinking about it. It probably was the alum mags and lower that shrank and not the polymer. |
I wouldn't. Material in/in.oF The OP posits a dT of 50 +-2 degrees or so (68-72 "room temperature", 20 ambient) so we're talking about the magwell (2.398 x .898) losing .0015 x .00001 inches. Not significant given design tolerances. Now, I'm not sure what PMAGs are made of (besides win) so we've got a range of possible materials above. Note that the plastics are shrinking MORE than aluminum, by a factor of 2 to 4.5. If anything they'd fit better in the cold. I'd go looking at the magazine latch. If that gets tight in the cold its likely to make seating a magazine difficult. The PMAG won't flex away from the latch as much. |
ha I had no idea most plastics had such high expansion coefficients |
I was under the impression that water was just about the only fluid that expanded when cooling down, due to the hydrogen bonding and the highly polarized molecules. I believe most fluids contract, or decrease in volume as temperature decreases. I know this is true for gasses (which, are also fluids). |