caliber specific cartridge are definitely better than muzzle installed universal sighter (the muzzle versions have significant variance just by rotating the unit)...
I've recently realized the NLT SIRT laser training bolt is a good bore sighter... I have the green laser with is easily visible in daylight (much easier to see than red)... it's a fun (pseudo) training device - but also a good bore sighter - I don't even use the cartridge bore sighter on AR15s anymore.
http://nextleveltraining.com/ar
BTW, you just reminded me... I have a 7.62x39 laser cartridge that I don't need anymore (recently sold all my AK stuff)... if anyone wants it let me know (free)... I also have a universal muzzle mounted sighter - but I wouldn't even recommend it for free (it will get you close, but there are better options).