9mm Buffer with 5.56 Upper?

I’m trying to setup a 9mm training gun that I can swap between 9mm and 5.56 quickly. I was wondering if there were any issues with just keeping the 9mm buffer in the rifle all of the time? The 223 upper will be a 14.5" M-4 clone.

Most of the time a 9mm buffer in a 14.5" .223 will run fine.

my 10.5 LMT will run with a 9mm buffer in it with all ammo EXCEPT for steel cased wolf. It short strokes. It works fine with the steel wolf with the can on it though.

I think the H3 is heavier than a 9mm buffer so I dont see why not.

I think the H3 and the 9mm are the same weight. The H3 has three tungsten weights but the 9mm has one solid weight.

My CMT H3 buffer weighs 5.43oz, I haven’t weighed a 9mm buffer but IIRC most are 5.5oz. The 9mm is a steel buffer and the head of it can move forward backward slightly and side to side. The H3 body is solid just heavy with 3 tungsen weights in it. The MGI rate reducing buffer is said to be 7.1oz and would work very well for 9mm, I’ve tried a MGI on a 16" .223 AR compared against an Enidine and honestly couldn’t feel much difference.

Slash on TOS makes even heavier buffers.