Last round pops out

[WHG]

I purchased a Rock Island 1903 serial # 338021 with a barrel dated 7/18. When I cycle test rounds through it the fifth round pops out when I extract the fourth one. In fact it is very hard to just put one round in the magazine and have it stay in. I've exchanged the follower and spring with one of my other 1903's, and even changed the follower with a 1903A3 one with the same results. Any suggestions as to why it's doing this?
Also, on the underside of the barrel just below the front sight is stamped C110 with a P under it. I've never seen this on any barrel before and was wondering if anyone new what it meant.

[Fred]

C110 would be the RIA Steel Lot Code for that barrel which was made in that month.

[louis]

I know this sounds crazy but is your cutoff switch in the correct position? I had the same problem once and then realized I had it incorrectly.

[Randy A]

Regardless of what position the cutoff is in a 1903 should "not" do that. Quite often this is from an incorrect fit between the trigger guard and action. Remove the bolt and take the action out of the stock, then install the trigger guard to the action (just by hand, won't need screws). Pay special attention to the inside of the action area where the well mates to it you will see how they fit together. Reinstall in the stock and the cause should be obvious.
Among a few other possibilities, I have ran into actions that Bubba modified for reasons unknown by grind/ file/ machining off the lips of the rails. I assumed for larger diameter cartridges or something, bad news then.

[chuckindenver]

i would need to see the rifle to assess it correctly..