First full auto is fun. Next since most of us cannot afford $10,000 +++ for full auto rifles, then the bumpfire style stocks work good for a reasonable price. The TacCom trigger does not seem to work good at all, per most of the reviews.
The SlideFire and BumpFire stocks do work well, with a little practice. The BumpFire is very similar to the SlideFire, at half the cost. It was not available when I bought my SlideFire, which interchanges between my M&P 15/22, M&P 15T, and DPMS AR-10. The 22 is the rifle that needs the most concentration on keeping a very minor amount of forward push on the handguard, so that it works correctly with the low recoil. The 556 and 762 have enough recoil that the stock works for almost anyone using it, even inexperienced users. They get lots of coaching and only a few rounds in the mag, until they are competent. That is how the instructor got killed teaching a young girl full auto with an Uzi, he used a full mag and she couldn't control the muzzle.
I agree that dumping full rounds is a waste, but I can easily control bursts of 3, 4, and 5 rounds. As far as accuracy is concerned, I can easily keep my 556 rounds in a 12 inch circle, at 50 yards at full auto rate, not just bursts You have to have a good stance and lean into the recoil, but you can control the muzzle. The AR-10 has a LOT more recoil and is not near as manageable. The 22 is very easy to control, but harder to keeps the full auto sustained. I have done slow motion video and the AR-10/AR-15 are in the 500/600 range of rounds per minute. The 22 has a slightly higher RPM count. I only have a few of the new Magpul 40 round mags for the AR-15 and 25 round mags for the AR-10. I do have a couple of 50 round drums for the 15/22.
I don't use the SlideFire often, but if you haven't shot full auto, then the bumpfire style stocks offer a near full auto experience, at an affordable price point. I have read a lot of negatives about "a waste of ammo/money, etc.", but until you've tried it, then you don't know what the attraction is. Everyone walks away with a smile on their face.
Bob