Tom, I don't think cutting a half inch off will make any difference at all. Just have the barrel chucked in a lathe, trim it and recrown the muzzle then, if needed, rethread it.
If you do have cycling issues after shortening the barrel that much (very unlikely) it will usually show up as a failure to feed. The bolt carrier needs to go rearward past the point at which the case ejects to pick up another cartridge from the magazine. Since any problem from the shortening of the barrel is most likely a drop in barrel pressure before the bolt gets a long enough energy impulse to fully cycle a feeding issue is typically the first sign of a problem. The next is a failure to eject followed by a failure to extract.
If you get cycling issues after shortening the barrel beyond the gas block you now have a rifle that is not getting a long enough duration to the gas pressure impulse and an adjustable block may help but the more likely solution will be to have you gunsmith use a tapered reamer to slightly enlarge the gas port to let a larger shot of gas into the gas tube.
Hope this helps.
Lance
