I need to purchase a bolt carrier for my semi-auto AR. I’ve used search and read up on the discussions of full auto vs semi-auto bolt carriers. But I’m left “wanting” mainly because the effect of reciprocating mass on felt recoil and accuracy hasnt really been discussed.
Here’s why. (I dont pretend to be an AR expert, but I understand mechanical things, physics, an guns in general)
When the round goes off the speed at which the BCG moves backwards is controlled by the combined weight of the BCG/Buffer combination and the recoil spring. If everything is designed perfectly, the BCG/Buffer will reach the limit of its travel just as the recoil spring is bottoming out.
So far so good. At this point, the recoil of a light bcg vs heavy bcg should be the same since the energy imparted on the BCG is being stored by the spring.
Where my question comes from is what happens when the BCG/Buffer slams forward abruptly. A heavier BCG/Buffer is going to “upset” the weapon more when it slams into battery than a lighter BCG/Buffer.
So . . my question/point is this. Wouldnt the majority of semi-auto users be better off with a semi BCG because of its lighter weight?
(I also understand the increased dwell time of a more massive bolt enhances reliability)
So lets refine my question. All else being equal. Will a lighter bolt result in a rifle that can be fired more quickly with greater practical accuracy?
Thoughts, comments, challenges are welcomed.
Are my assumptions wrong? Rip it apart.
I think a light fast bolt group will give you “bolt bounce”. This is where the bolt closes and then bounces out of battery a little bit before returning to battery.
It’s not a good thing. Heavy as reliably possible is generally the best way to go.
Can anyone identify this weird bolt carrier? I have never seen one like this. The ports on the side have 3 holes, rather than the standard 2. The front of the bolt carrier has a lip going around it and isn’t flat. Also the inside area where the bolt moves around is not shiny chrome and looks as if it dremeled out.
it looks like a copy of a LMT enhanced carrier. the holes in my enhanced carrier look a lot more true… rounder. also the inside is chromed not dremeled. but i cant really see the inside so well from that pic. hrm.
Guns won’t blow up from bolt bounce. It causes a machine gun to choke however. The A/R can’t strike the firing pin out of battery. It’s impossible. I think the early M4 full auto jobs with standard carbean buffer would bounce bad and stall out the gun in full auto.
The advantage for a semi gun is really the shrouded firing pin… not the extra, negligible weight. The shroud cocks the hammer instead of the firing pin. That’s the way the gun was designed. The ramped carrier is a trick to get the bolt group to stop moving incase the disconnector fails.
It is my understanding that the lighter BCG/Buffer/adjustable gas block will give you less felt recoil because less mass is moving and it takes less gas to move the lower mass. One of the problems is if the gun is over gassed it could cause the spring to bottom out and have the BCG hit the back of the buffer tube. To control this you would need an adjustable gas block to reduce the amount of gas.
Using a heavy BCG/buffer and a light BCG/Buffer/adjustable gas block are both dealing with the same problem, the rifle being over gassed. One deals with it by increasing the weight thus slowing everything down and the other reduces the gas thus slowing everything down and if you have an even lighter BCG/Buffer it will take even less gas.
The problem with using an adjustable gas block is it is another part that can get out of adjustment either not allowing enough gas to cycle the action or giving to much gas and causing the BCG/Buffer to bottom out the spring and hit the back of the buffer tube.
The problem with the heavy BCG/Buffer is it can increase the felt recoil, which is not a very big issue when reliability is a life and death situation.
This is why you will see duty guns with the heavy BCG/Buffer and 3 gun ar’s with the light BCG/Buffer and adjustable gas block.
That makes a lot of sense. Thats why I specifically focused on accuracy and controlability.
I understand that more massive bolt may help the soldier. But it sounds like for someone who just wants a nice shooting target, plinking, or 3 gun AR, a lighter bolt, with the correct spring and a normal gas block is the best way to go.
One other factor you didnt take into account is using a heavier spring with the lighter bolt, rather than an adjustable gas block.
Generally speaking, for a given recoil impulse, you can swap reciprocating mass for spring tension. Compare the spring on a Kahr PM9 to the spring on a Glock 17. The 17s spring is much softer because with a heavier slide, it doesnt need as stiff a spring.
Going with the light BCG/Buffer and regular gas block with a heavy buffer spring could work, but it would still have more felt recoil than with the adjustable gas block because BCG/Buffer is still going to have to over come the stiffer spring thus needing more gas and giving a bigger recoil impulse. In this case it would be just as good to use a standard BCG/Buffer and regular spring (or what ever stiffness spring is appropriate for the amount of gas your getting) especially on the cost side.
Changing the weight of the BCG/Buffer/Buffer spring is just treating the symptoms of the overall problem of the rifle being over gassed. The adjustable gas block solves the problem of being over gassed and then allows you to go even lighter than normal which gives you the smallest recoil impulse possible. The adjustable gas block gives the most reduction in recoil impulse besides a good break.
Ideally you want just enough gas to keep you from short stroking while you are limp wristing the rifle. If it is a duty gun then you are going to want some more gas (not too much) to make sure that everything works in all positions even if you get a slightly underpowered load.
One of the other problems with the light BCG/Buffer/adjustable gas block is it is going to cost you over 300 dollars probably closer to 400 in parts alone. Then you have to be able to change the gas block, which not everyone is comfortable with or has the tools to do. Putting in a heavy buffer is relatively cheap and anyone can do it.
LMT Carriers also have an altered cam slot timing and the middle of the four rails is missing(This is some kind of Sand Cut). Either this is an early generation or someone else made an incomplete knock-off.
Thanks again. My main drivers in putting up this thread are 2 fold.
I like to understand things, and there hadnt been much written about the benefits of the semiauto (AR) bcg vs the full auto (M16) bcg. Everybody only talks about the benefits of the M16 bcg when firing full auto. Well, this one isnt going to fire full auto.
I’m about to order a bcg from LMT and am leaning towards the semi-auto for the reasons above and wanted some validation. I’m not looking for anything exotic.
The 16 carrier isn’t beneficial for full auto. It’s mandatory.
The benefits for the standard AR are really in the firing pin shroud. The weight diff between the regular carrier and the emasculated carrier is completely negligible. Thus, there’s really no need to buy anything other than the original part… the M16 carrier.
If you’re wanting fast follow up shots, a comp is the way to go. Installing frooty springs and stuff is just asking for problems. If you start getting too much bolt speed, your bolt will start failing to lock back and strip rounds from the mag and stuff.
A nice compensator sounds like the way to meet your needs.
If reliability is secondary then I would go with a reduced mass JP BCG and an adjustable JP gas block setup and a light-weight spring and tune it where it barely kicks them out then slap a comp on it and you can sit there rapid-fire with narry a wiggle.
This is for a range toy only.
Otherwise, I have found the FSC556 from PWS to be an EXCELLENT comp that isn’t too “blasty” and won’t blow muzzle-blast into your sight picture.