Tippman A5 and C98, Spyder MR1. The A5 is "better" but you get what you pay for, right? More features, to some, means a better gun. The C98 is basically the SAME gun as the BT4 Combat, internally. The BT4 is nicer looking in every way though. MR1 is a love it or hate it gun, from MY experience. I've seen many problems with MR1's that I've never seen happen to a BT4 but I've never owned one so...
BT4 Combat, A5 Flatline, Halo Loader, Rear Velocity Adjuster, Upgraded Spring, Remote Line and a Tactical Flashlight / Foregrip combo straight outta Hong Kong!
The BT-4 Communist!
I also rock an all black Proto Matrix Rail with Halo, On/Off ASA, UL Trigger, Trinity clamping feedneck, 14" JJ barrel- soon to be a 14" or 16" Freak and maybe a drop.
You must get a new barrel, like any paintball gun. Depending on the barrel you choose you might have to buy the spring kit because the stock spring is pretty soft. It's probably good for most barrels unless you do something dramatic like put a Flatline or an 18" barrel (I don't recommend a barrel that long though).
Cleaning it and oiling it can be done in 5 minutes
Feedneck is a little wobbley with heavy hoppers
The ASA is pretty ugly... ;)
I bought this gun for MilSim purposes and it does the job, very well. Honestly, I didn't come here to review the BT4 Combat specifically because there are many reviews here that I agree with and I think do the job. Mostly, I wanted to settle the A5 Flatline + BT4 debate I was reading so much about because I liked the concept of flat trajectory shots.
Well, I got a BT4 and put the Flatline on it right away, I never shot the stock barrel, no point. So, first of all, anyone who says they don't work well together didn't bother to make the right adjustments. The Flatline is VERY temperamental and it has a Sweet Spot, which happens to be right around 300 FPS- AKA legal limit. Like I said earlier, I had to change the spring. I bought the MEDIUM / Grey spring off a friend for $2 and the BT4 gained 60 FPS. Since I was already shooting in the 280s it was more than enough to give the Flatline the speed it needs to put the backspin on the ball and shoot a "flat" shot. By "flat" I mean that you have to remember you're shooting paintballs. They are NEVER perfectly round and will not go perfectly straight- at least not every shot. No amount of backspin or barrel length will change that so, yes, even a Flatline barrel has some variation in it's groupings. But it does deliver on the flat trajectory once you've tuned your gun properly and that makes using your "iron sights" really nice.
I can only really think of one negative thing that comes from putting a Flatline on a BT4 Combat. The barrel shroud extends to the top of the gun and so to take it apart you have to remove the 2 screws that hold the bolt in place just to remove the feedneck off the back of the gun. It's not major but it's an inconvenience to me ;) other than that, I can think of no reason why someone shouldn't try this combo.
I think that if you are buying or bought a BT4 Combat and WANT a A5 Flatline because it looks super bad*ss but aren't sure it will perform than have no fear. It works just as well on a BT4 as on an A5 and maybe even better... because the barrel does come up so high on the gun body, the iron-sights actually line up with the ball's trajectory almost perfectly. Also, I feel that the Flatline doesn't like long bursts or full-auto; it looses a lot of accuracy after the first shot. So, in my opinion, don't bother getting an E-Grip for that reason and they are finnicky and unreliable. All in all, I think BT4 Combat is a good gun for the little money it costs and for a little more money you can make it a great gun.