The De Lisle carbine is .45 APC, same as the m1911. Also it was known as being not very accurate. Pistol ammo + low accuracy = something you probably shouldn't put a scope on.
There's a lot of things I want to say here, because I happen to be an American gun nut who has a lot of experience with this subject, but I'll try and sum up.
Firstly, no matter what you're mounting it on--even a literal pistol, impractical as that would be--using a scope will be more accurate than using iron sights. This is because while the bench accuracy of guns may vary, it's only half of what determines whether you hit, the other half is the user. Simply put, if you aren't aiming at precisely the right spot, you're less likely to hit, and scopes make it easier to aim perfectly.
*Secondly, I only want to put a scope on the De Lisle because my character has +3 skill in heavy rifles--which seems to mean "scoped weapons", considering it's used for recoilless rifles, the springfield, and a scoped Gewehr--and no skill with any other gun. Though unrealistic, this is equivalent to a guy who's never ever fired a gun with iron sights wanting to put a scope on his gun, because he has significant experience using scopes. Sure, that wouldn't be nearly as important as my character's ridiculous specialization, but it isn't entirely unreasonable either.
Thirdly,
this is a game, ran by a guy who knows less about WWII than many of his players. It's a game where sniper rifles and recoilless rifles use the same skill, because they're both named "rifle", despite the M1 and springfield using entirely different skills. It's a game where the Springfield has 2, 3, or 5 lethality depending on range, despite being ballistically very similar to the M1 which has 3 regardless, presumably due to the scope. It's a game where pistol whipping a guy is just as lethal as stabbing with a basic knife. Slamming a scope on a gun that a scope doesn't make sense on will probably not affect anything in any particularly significant way, and if it does, it likely will not affect it in a realistic fashion.