So I'm perusing my ammo supplier and I notice MagTech has 85g jhp, and it also says +p. Is this just hotter, or do a need a designation on the gun to shoot it?
+P means that it is "hotter", that is it generates more than the SAMMI-rated pressure. +P+ is even higher pressure. You should check that your firearm can handle it. For example, Ruger warns against using +P with the LCP.
If it's a newish gun it should be +P rated, but the manufacture could say for sure. You'd have to look up the spec (if there is one for .380) to see how much, but it will be a higher PSI.
There is no SAAMI specification for "+P" in .380. So anything marked as such could be anywhere from a little bit over to WAY over (as in borderline-unsafe) the SAAMI pressure specs. Buyer beware.
You'd prbably be ok if one plus p slipped into your ammo pouch and you didn't know. Then again it may blow up your gun, not likley one would blow your gun but it -could- happen. Now, ongoing usage of plus P in a gun not rated for plus P is just a ticking time bomb.
Magtech, as well as many many other manufacturers, do make 380 that are not rated for plus P, so I don't think you'll have a hard time finding ammo.
If you really need a +P .380 load, why not just go buy a 9mm? As someone pointed out, read what the gun manufacturer says about +P loads in their product. Seems like it would really be quite snappy shooting from a little .380 pistol.
Ditto what shooter said. Also add that what might be just a maybe-not-so-good idea in a locked-breech .380 could (potentially) really ruin your day in a straight blow-back one. Just my $0.02, but if they were my hands and eyes I'd pass.