After Wade dies, when the squad is attacked by the German half track, when it is shot with the Bazooka. Two of the soldiers jump up and fire at the Germans. One is Melish, but the other one is Jackson, and he is armed with a M-1, not his sniper rifle.
Miller asks "Who's going left?", after some hesitation Jackson says "I'll go" so Miller tells Upham to "switch off with Jackson". They exchange gear so Jackson has Upham's M1 instead. Which is why Upham is looking through a scope in the first place.
do you really think jackson and Uppham would keep each others guns for that long without giving them back? and anyways, when they're making the german dig graves, you see uppham with his m1. i watched it yesterday to make sure it was jackson shooting the m1 and sure enough, he has his scope protectant tube on his back and he is shooting an m1. this is a slipup.