Hello there:
I don't want to rain too much on your parade or the T99 longs, but using a donor lower section of a T99 short wont work. The butt plate of the T99 long is longer than a short's is.
From T99 long SN# 838...0 series..... length of butt plate = 4 11/16" ....from top of butt plate to center of screw = 3 5/8"
From T99 short SN# 53830...0 series..... length of butt plate = 4 1/2" ........from top of butt plate to center of screw = 3 7/16"
Both are from Nagoya of course and the T99 short is a cavalry rifle. Just to confirm, I checked a series 4 SN# 96681 as to butt plate size and the regular and cav. short T99 have the same dimensions.


I don't think you can counter bore the T99 since it's chrome plated. I heard a guy at the Knob Creek MG shoot asking one of the smiths out there about shortening a AR 15 barrel. The smith said "sure, as long as it's not chromed, too hard to work".