The mags that don't leak while filling have black or red O-rings in the fill valves.
TM's don't have those, so they leak on filling. Totally normal. TM's reasoning is so you can tell when the mag is full; liquid gas (as opposed to gas vapor) will leak out when the mag is full (it leaks much more once full).
Although, with the O-ring fill valves, you just put your ear on the propane cylinder and you can hear when the mag is full.
You can buy moondog O-rings to put on your fill valves, but there's not really an advantage other than not freezing your hands.
Push down slowly, make sure the can is lined up straight with the mag, don't wiggle it. The fill valves are made of tiny, very weak brass parts and can break easily when angular force is applied. Get gloves, or hold the mag near the bottom, liquid propane is REAL good at causing frost burn.
MAPP is a mix of liquified petroleum gas and propylene. Smells like garlic asshole, although not as bad as Rona brand propane. Dunno what Rona puts in there but that shit is NASTY.
The tall blue bottles are propane, but tend to be cleaner than the green 1lb camping fuel propane tanks. Don't know if it really has any effect on the seals, though.
If you LIKE running just the pistol, then do it till you're sick of it lol
You'll still get great accuracy and range from it, but you're basically playing hard mode outside of CQB.
Unlike in real steel, barrel length really doesn't make any difference in range or accuracy. You'll get the same range and accuracy out of an MP5A3 as you would an SR25.
But if you're in and out of CQB often, what you're looking for is a short gun with a stock. Small enough to still be used in CQB, or tossed to your side to switch to your pistol, but stable enough that you can use it to engage people at 200+ feet, and light enough that it won't slow you down.
Aesthetic preference aside, something in 9mm would probably suit the light and agile playing style; MP5s and the ASG scorpion evo are the top two that come to mind.
KSC MP7 or KWA vector if your want to go for a gas SMG.
ARES honey badger is all plastic, yet sturdy, so it's light weight and fairly short, but the AR mags are twice the width of MP5/evo mags.
If you're looking for something in the M4 family, you're not cool until you have a MK18 MOD1 lol