a muzzle brake on an bullpup such as RDB will create unpleasant experience for whoever is shooting it. if the plan is to run a red dot I would go with a linear comp like the troy claymore or kaw valley linear comp. it will direct blast forward at the expense of slightly increased recoil. the RDB will not be accurate anyway so it's not going to run a higher mag optic. a muzzle brake serves zero purpose on such a rifle. having owned keltec products in the past I would strongly suggest saving your $$ and put towards an IWI, CZ or bushmaster ACR all of which are 100x better rifles. further, keltec has poor CS and will only work on products for original owner, thus the alleged "warranty" disappears if you sell the rifle. their products absolutely tank in value. I commend keltec for coming up with interesting designs including a downward ejection system, but their execution of design is lacking. there's a reason why one sees so few CZ 805's on the used market; because they're excellent. same with the IWI SAR or X95's.