Ok this might have been covered already however my search did not yield conclusive findings. I came across a lot of talk and speculation about what happens when you cut ar500 with a torch. If it will weaken the steel enough so on and so forth. Here is what I found.
I cut a 1/2 " hole in my plate with my torch, mounted it to a 4x4 post and went out and shot it today:
Plate: 3/8th AR500
Range: 100 yards on the dot
Ammo: 62grn xm855 green tip
Rifle 18" spr 1/7 twist barrel.
I took my first shot right next to the bolt, and then down the plate. The first shot went less than a 1/4 an inch deep but it did do damage, the 2nd shot 3 inches down although it looks like damage really was not even noticeable by touch(maybe a slight impression). And the latter bullets all just scuffed off the paint and did nothing to the surface.
So although this is not scientific here is my opinion. If you are cutting holes in these targets to chain hang or mount for LONG RANGE TARGETS 500 yards or more.. I would not hesitate to use a torch, especially a plasma cutter. This steel is very tough and 62grn ball is only affecting the plate within a few inch circumference at 100, even magnums are not going to do any damage on the weekened cut area out at 500+
Now if you are using these plates for close up rifle shooting I would recommend having them cnc waterjet cut. I should have taken more pictures but we also had targets setup out to 500 yards and this same gun and ammo, as well as .308 150 grn FMJ was not even dimpling the steel from 100 and out.
I will be shooting these same plates with 300 wsm 208Amax this weekend and will get some results up from that soon.
Sorry for the bad quality pic but it got dark on us.
I make my targets with 1 inch thick AR500 all cut with plasma, they have several thousand rounds put
to them from 223 up to 416 even my 50 ak and 510 whisper dont do more than a pot mark even with steel core
I like the re-bar design there.. yeah I found a really good deal locally on 12"x12" plates.. it is 3/8 though.. But in the picture above 2 of those hits are 150grn FMJ at 100 yards with a .308 and not even a scratch.. I usually shoot long range and will most likely not shoot plates any closer the 300 yards out to 1k from here on out.. So 1/2 is over kill (and 1 inch is way over kill) as the largest caliber I have is 300wsm.
I just did a lot of reading on other sites and people talked a lot about how if you cut them with a standard torch it can weaken the steel... So I thought I would put it to the test.. and as I said above you can get away with it.. but wouldn't do it for up close rifle rounds.