I can fit 5 in my Axis magazine, which uses the same upper housing as the 110-series magazines, and don't have a problem seating it with the bolt closed. Whether that holds true on the 110-series guns with the metal bottom and separate bottom plate I don't know - don't own one of those.

Putting a spacer between the bottom metal and the stock is going to create more problems than it solves. The "reason it would be hard to seat is because there would be no give left in the magazine's spring/follower with 5-rounds in the mag. If you space the mag down to solve that issue and you'll end up with a feeding issue as the bolt will likely no longer catch the top round in the mag to strip it off and chamber it - especially with the smaller diameter cartridges.

You said you intend to use this rifle for competitive shooting. Well, I don't know many folks who half-ass jerry-rig things on their competition rifles, at least not if they're even remotely serious about it.