A. Reliability Kit:
B. Bolt Weight:
What does the weight do?
The weight reduces bolt velocity, all other things held constant.
Why does bolt velocity matter?
Optimized bolt velocity means the powder burns more fully before the case is extracted, reducing fouling, noise and gas blowback to the shooter. It makes the rifle feel smoother, and reduces battering of the bolt carrier and lower receiver (especially when combined with a buffer), and also increases consistency shot to shot with the cartridge in battery longer.
Should I always use a bolt weight?
No! Use of a bolt weight is not recommended with some combinations of low power ammo and barrel lengths. A good rule of thumb is that if cycling is borderline, a bolt weight should not be used.
Testing bolt weight function and/or tuning: Load a single round into the rifle with a magazine and fire (in a safe and legal manner). If the bolt locks back, repeat for at least 10 shots to verify consistency of bolt hold open. If your rifle is not equipped with a last round bolt hold open that works with the bolt release lever (no, the CMMG/Black Dog mag follower hold open does not count), check for the trigger resetting. If all 10 rounds result in bolt hold open and trigger reset, great. If not, either remove the bolt weight (Heavy Duty) or reduce the weight of the adjustable weight.
If using the adjustable weight, we recommend starting with the aluminum internal weights installed, and then shooting single rounds, adding weight until you do not get bolt hold open, and then reducing the weight enough to pass the “10 round test”. After passing the 10 round test, it is advisable to lighten the weight somewhat further to allow good cycling as fouling accumulates. Immediately clean the rifle if any cycling issues are noticed to restore proper function.
Which weight should I buy?
You shoot primarily high velocity ammo in 8″ or longer barrels and want a simple solution without extra cost or complexity with the benefits of reduced bolt velocity.
1) You shoot a variety of loads from subsonic to hyper velocity and want to be able to tune exactly to each load for optimal cycling and ultimate accuracy and reliability.
2.) You shoot uppers with short barrels.
3.) You have a registered full auto lower and want flawless full auto function.
What variables are involved with tuning bolt velocity, and why do I care?
There are many variables involved in a blowback system, and for the sake of the majority of this discussion, we’ll assume that bolt mass is held constant (more on this later). Listed roughly in order of significance:
Ammunition: This one’s pretty obvious, the more powerful the ammo, (generally) the more impulse that pushes back on the bolt.
Without getting too far into the weeds on the physics, when a force summed up over time, you get a quantity known as “impulse” (in the weeds: impulse is the integration of the force with respect to time, or the area under the force-time curve). Simplified, if the force is constant, the impulse is force multiplied by time. This why, for example, you can floor your car and get to 60mph in 10 seconds (high force), or you can gently accelerate and get to 60mph in 30 seconds (low force). Either way the impulse is the same, and it resulted in you getting to 60mph, but the forces were different as was the time that that force acted over. A small force acting over a long period of time can result in the same velocity as a large force acting over a short time. Remember about impulse for later.
In the AR22 as in other blowback semi-autos, the impulse is caused by the pressure generated during firing pushing back against the case, and thus pushing on the bolt, causing it to move and cycle. Something like a CCI Velocitor (40gr, 1435fps) has a much higher total impulse than, say, a CCI Quiet-22 (40gr, 710fps). With the same bolt mass, the Velocitor will cause a much higher bolt velocity.
What does this mean for me, the humble 22lr shooter? It means that at some point, if you use ammo low enough on the power spectrum, your gun will cease to cycle fully or at all. Conversely, if you use high powered ammo, you may get an undesirably high bolt velocity (which is then corrected by adding mass to the bolt…more on this later)
Barrel length: Again, we’re oversimplifying here, but remember impulse and how it gets larger if the force has more time to work? This is why longer barrels lengths result in higher velocities — the force has more time to push on the bullet. Likewise, the bolt will gain more velocity with longer barrels.
What does this mean for me, the humble 22lr shooter? It means that the longer the barrel, the lower power ammo you can cycle given the same bolt mass. For example a 4.5″ upper may be able to barely cycle federal bulk pack (36gr 1260fps) while a 16″ upper may not only cycle but have an undesirably high bolt velocity (which is then corrected by adding mass to the bolt…more on this later)
Springs: Springs have a smaller effect than you may initially think — at the instant the cartridge is ignited, there are several forces in play. There is the force from the recoil spring and hammer springs that are holding the bolt back, and then there is the force generated when the cartridge ignites. Imagine a minivan stalled on a railroad track with a train quickly approaching. Now imagine that an elephant is standing outside the van pushing on the van in order to try to hold it still when the train hits. That elephant standing there is like the spring — it is are imparting a force on the van that seems pretty large if you don’t consider the train, but compared to the train, the force doesn’t amount to much at all. If you increase the spring power (say by putting a big dodge ram diesel pushing against the minivan, you can counteract some of the force of firing, but it’s still pretty minimal in the scheme of things.
Once the bullet leaves the barrel, however, and the force from the firing goes to zero, and the springs begin slowing the bolt down gradually over the travel of the bolt. Swapping to heavier springs can influence how hard the bolt hits at the rear of travel, but they don’t slow the bolt much initially.
What does this mean for me, the humble 22lr shooter? It means that unless you’re shooting very low powered ammo that is barely able to cycle the action, you’re not going to see much change with reasonable changed in spring strengths. You’ll probably read on the forums at some point about reduced power hammer springs being needed to cycle with subsonics and/or short barrels, which falls into the “barely cycling” or “almost cycling” category.
Bolt Mass: Finally, we can discuss bolt mass. If all things are held constant, and you add mass, the bolt velocity drops. So adding a BoreBuddy weight results in lower bolt velocity with a given ammo, barrel, and spring combo. Optimizing bolt velocity smooths out the operation of the system and helps with reliability.
What does this mean for me, the humble 22lr shooter? Added weight and thus optimized bolt velocity means the powder burns more fully before the case is extracted, reducing fouling, noise and gas blowback to the shooter. It makes the rifle feel smoother, and reduces battering of the bolt carrier and lower receiver (especially when combined with a buffer), and also increases consistency shot to shot with the cartridge in battery longer.