Is it me or the gun?

I have a High Standard Citation, 5.5 bull barrel, open sights.  In the last few weeks I noticed that the groups open up considerably after 20 to 30 rounds.  I thought it might be me so started shooting off a bag - same results.  The first 20-30 shots are a tight group, by the time I get to the 100th shot, the groups are two or three times the size 2" groups to 6" groups.  I've checked this three times now and it doesn't seem to matter if the barrel and chamber are clean or not.  Any thoughts on what might cause this?  Thanks, Dan

im guessing chamber.

https://www.brownells.com/handgun-parts/barrel-parts/barrels/match-barrels-for-high-standard-prod7928.aspx

When the groups open up, if you brush it out and immediately go back to shooting do the groups go back down?

Slow fire, rapid fire, ammo used, and other variables?

Blackbird wrote:Slow fire, rapid fire, ammo used, and other variables?

I'd say that is closer to "timed fire" - probably 20 to 30 seconds between shots, 5shot groups.  I've used CCI Standard, Aquilla Target and Pistol Match and the results have been the same - groups open as I continue to shoot.

What do you do between these 20-30 shot sessions? 

Do YOU fatigue?

Couple of things that might help determine the problem. Do you have the older High Standard with the push button to remove the barrel or the newer type that has the cap screw? If you shoot a few rounds and accuracy is "normal" then not continue shooting for a while will accuracy return to good or continue to degrade? After shooting and accuracy falls off what do you do afterwards, clean the gun and have it ready for the next shoot? Do you remove the barrel to clean the gun.