The top BCG is a full auto as compared to the “neutered” BCG at the bottom. Notice the difference in the rear.
The weight of a Bolt Carrier Group, or BCG, in an AR15 platform has been modified from the original weight of a full-auto BCG in an attempt to prohibit full-auto conversion of otherwise non-NFA weapons intended for commercial civilian use or sale. Engineering and machining modifications have evolved the BCG as the weapon was made available to the civilian commercial market in semi-auto from, per the National Firearms Act. It has become an accepted practice of milling the tail end of a bolt carrier, or lightening the carrier, in the area where the full auto sear is tripped by the carrier during function/cycle of the weapon for proper full-auto function. This modification reduces the mass of the carrier. The weapon should still function, though the debate about whether or not it is still as reliable as original full carrier weight does exist.
The force required to strip the first round in a 30 round magazine is increased when the weapon is dirty, dry, and fouled. Any reduction in weight of BCG as described may prohibit or lessen the probability of properly chambering a round, and bolt successfully moving into battery. The AR-15’s basic design prohibits it from firing out of battery, which would be considered a catastrophic failure. Also, some competitors in shooting sports try to gain an advantage with a reduction in weight/mass of BCG called low mass carriers.
This has driven rifle accessory, including Bolt Carrier manufacturers anxious to sell products when the market is down, to swiss cheese the hell out of a perfectly good bolt carrier design because they know the consumer will pay for it, even at the expense of function, reliability, and even safety. For example, aluminum bolt carriers, which have been known to fail and include a “do not use in a duty weapon” warning.
Reliability is usually affected, and whether it is seen in competition or practice at some point the rifle fails to cycle from either being over-gassed, or under-gassed, or removal of standard duty weapon components. There can be a place for said performance competition accessories but like most performance-based mechanical assemblies it takes more effort and more due diligence on maintenance to maintain reliability. The small reduction of the muzzle flip as a full weight carrier arrives in battery is minimal, and the level of shooter required to warrant that mitigation is a small group of elite shooters as a whole.
Lots of time, money, and effort has been wasted on modification of that area of the weapons platform, and it often exacerbates any function issues a person is having with a particular set up.
Most often it is not a drop in type modification, without other manipulation of variables as discussed, on gas system length, gas port size, buffer weight, spring rate, and so on. Even excessive force of the hammer cocking spring can affect reliable function.
To build a reliable low recoil competition platform, an effective muzzle brake, Jerry Miculek’s Compensator, for example, is a necessity. A quality trigger such as an AR Gold, HiperFire, or Geissele and an adjustable gas system such as the MGI style Adjustable Gas Tube like we use on our Barnes Precision Machine Three Gun Match Carbine model will limit recoil from over gassing of BCG – is a good start.
When using a full weight bolt carrier group, along with an effective muzzle brake, an adjustable gas system, and a quality trigger group you will more than compensate for any perceived difference from a lightened bolt carrier group without sacrificing reliability of safety. The return mass and energy of full weight BCG provides proper reliability to strip rounds off a fully loaded, dirty, or dry magazine with the ability to tune the rifle to your match ammunition recoil expectations as you see fit, without affecting reliability.