The V2 has a fixed A2 front sight post gas block with a shorter railed forearm.
The V4 uses a low profile pinned gas block that sits underneath a longer railed forearm. They then use a removable clamp-on front sight post.
If you plan on running any type of magnified optic, go with the V4 and you can remove the front sight post and rear sight post and have a full flattop. OR, go with the flip-up sight version.
The midlength gas system is 2" longer than the carbine system, making it a little more reliable because it slows down the timing of the gas system and reduces bolt speeds. The recoil pulse is more smooth and less of a "pop". I would recommend midlength for many shooters, as it is an excellent option for having a longer rail with smooth recoil.
I have shorter arms and a very large chest, so for me and the way I shoot, and carbine length gas system is a better fit due to how I hold my rifle. My ideal DD is the Mk18, but non-NFA, it's the M4V1 with the extra real estate in front of the FSP for mounting a light.
As far as lightweight (LW) barrels, there are pros and cons. The pro is of course that they're lighter. You only shave off about 5oz of weight, but as it's often said, "ounces equal pounds, and pounds equal pain".
LW barrels have a couple downsides. The downside of having less weight is that you have more recoil effect on the gun, requiring you to employ more force and technique to drive the gun and control it. Another issue is that you are dealing with a thinner barrel that is less rigid and less resistant to heat. The barrel will heat up faster, and barrel life is much less than a standard barrel. CHF barrels have a better barrel life as it is, but you'll still have a lower service life (though not as low as a cut-rifled barrel). The problem is that if you start getting aggressive with the gun and you heat it up, the barrel's accuracy will start to deviate and your groups are going to open up due to the heat. Your accuracy will continue to suffer as long as the barrel remains hot.
If you just plan on plinking and target shooting, and your normal range trip is only 90-120rds, and you shoot less than 2,000rds per year, the LW barrels are great options. If, however, an average range trip is 250+rds and you consistently heat your gun up hot enough to evaporate water on contact or create a 2nd degree burn immediately, and your annual round count is pretty high, then you need to go with the standard CHF barrel.
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