I got a good deal on a basic TYR GPC from a friend and plates needed to be ordered. The shooter’s cut plates I want come in 10 x 12. Can 10 x12 plates fit properly in a carrier that is sized for 10 x13 or will they shift or not be positioned correctly in this carrier? Will soft armor backer IIIA help?
I think you will be fine. If you have the GPC already, look at how the plates are secure inside. You are good width wise, you just need to make sure you can make up for the extra inch vertically. Looking at my PICO MV, there is a strap on the bottom that holds the plate vertically in addition to the Velcro closure on the bottom. If you have something similar, you should have not issue.
They should fit fine- if there is any play, you could cut a piece of foam to take up the extra inch
It’ll fit, the backer will still be useful; you’ll lose the principal advantage of the shooters/operator cut plates as far as being able to shoulder stuff fairly low in standing positions, and relying on padding or the internal vertical strap to position the plate vertically is a little less ideal if you’ve moving a ton, but as a rule running with plates is going to suck anyway (and yes, the soft armor does help with that).
That said, there are some lighter, arguably higher performance for $ (either cheaper or multi-hit capable at lighter weight) plates in the more standard shape, also available as triple-curve setups that would be worth looking into over the relatively constraining single curve 10x12 shooter’s cut plates if you want to go with the larger plate carrier. If you’re using an in-conjunction-with setup, the 3/4" thick standalone ceramic plates are a touch heavy, thick, and poorly curve-shaped considering that you’re looking at a 14lb slick armor setup with even the light Lvl4 plates and HDPEsoft armor… minor differences, but I’ve spent a lot of money trying to get that last bit of optimization onto my gear.
FWIW, I have a SAPI cut soft armor sewn-in Eagle SPC that I do run with 10x12 shooter’s cut plates (the cheap but heavy MSA set); it does work, but I’m saving maybe half a pound over a simpler lightweight plate over that armor setup. I have a standalone plate carrier ready setup, which trims some of that weight, but it loses all of the flexibility of running soft armor (pistol-only shoot house stuff, or better concealability slick) and my new setup will only make sense as a ‘best case’ setup if rifle threat is the determining factor (but gives up flexibility, load carrying comfort, don speed, hydration carrying simplicity, and cost in order to be maximally concealable with rifle plates).
Grab some 1/2" foam pipe insulation at Home Depot and cut two sections to fit the top and bottom of each plate. Slide it on. Works great.
Used a similar plan with a GI sleeping mat. I also support TehLlama’s point about getting it secure to reduce the gut check when you are already huffing and puffing in a sprint.