Internals same as 98 custom.
Accepts a-5 flatline.
Not a cold weather gun.
the bt-4 assault looks and feels good in the woods. its well made. the insides are identical to my 98 custom in-line hammer, valve, and bolt. the assault barrel was impressive with its accuracy. it has the barrel adapter like the a-5. you can buy different adapters that will accept other gun's barrel threads, including the flat line. I have the flatline barrel on my assault right now and i love it. one advantage of this gun compared to an a-5 ia if you break a ball, you slide the hopper back and run a squeegie through it. but there has been a couple of times i had to shake the gun to get balls to feed so whatever you like i guess. i took the stock off the assault and the sight rail and put them on my 98 custom.
On the down side this gun uses a lot of co2. When I got it i could only shoot about 300 balls on a 12 oz tank. when its cold outside tis gun was using so much gas it was making the tank freeze faster than i had ever seen a gun do. i think it was 44 degrees that day. i was shooting one ball every 4 or 5 seconds at a target. I think i have fixed that problem though. i took both my guns apart side by side( the 98 and the assault). I noticed that the drive spring on the assault was a 1/4'' longer than the spring on the 98. So i cut off a 1/4'' of the spring. the next time i played it did perfect.
This is a good gun if you fix the gas hog problem but, I would recommend you go ahead and buy a tippmann 98 or a-5 and just mod it yourself. If i could do it again that's what I would have done. That's why i have the 98. I bought it after I bought the assault and not for a backup.
THIS gun does freeze just like any other metal gun without an expansion chamber would freeze , it is metal!!!!
A expansion chamber fixes that issue. I have owned one for about 9 months, Played straight through the winter. Operates fine in temps as low as 37 degrees. This is with high rate of fire. Great reliable gun with plenty of upgrades out for it and a bunch on the way. I have been told that a cyclone feeder will be coming out for it also