which AR buffer/spring do I need?

I don't think so, unless it has reliability issues. It'll function pretty much the same.

A rifle Buffer is 5.15oz and uses a longer tube and spring.
A 9mm Buffer is about 5.4oz

Carbine buffer weights are as follows.
Car= 3.0oz
H= 3.7oz
H2= 4.6oz
H3= 5.5oz

There are a couple things that play into how heavy your buffer should be.

Who makes your barrel? What type of Bolt Carrier is in the gun?

In the end though you should run the heaviest Buffer you can.

shoot it and see where it ejects, you want 3 oclock-5 if i remember correctly. if its ejecting in the 12-2 range then all you really need to do is up your buffer weight, go to an h2 or h3 or possibly buy the tubbs spring I had to do both on my 6.8 which was slightly over gassed. If it starts short stroking, then you need to go down in buffer weight.

A 20" rifle upper will generally work with a carbine or H buffer in a carbine tube, based on a fair bit of personal experience.

The spring should simply be the appropriate spring for the tube and buffer length you have - rifle spring for rifle tube, carbine spring for carbine tube. 99% of the time with non-SBR setups you should not get any sort of custom spring with special parameters.

Z-Michigan said:

A 20" rifle upper will generally work with a carbine or H buffer in a carbine tube, based on a fair bit of personal experience.

The spring should simply be the appropriate spring for the tube and buffer length you have - rifle spring for rifle tube, carbine spring for carbine tube. 99% of the time with non-SBR setups you should not get any sort of custom spring with special parameters.

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I concur, I've made this swap several times in the past and never encounter any problems. I do have H buffers in most my carbines, but I've seen them run fine with the standard carbine buffer also.

Buffer weights can vary over .1oz from buffer to buffer. I've had good luck with the ST-T2 buffer in carbines and pistols

Carbine 3.0 oz -> 3 steel weights
H buffer 3.8 oz -> 1 tungsten weight
H2 buffer 4.6 oz -> 2 tungsten weights
H3 buffer 5.6 oz -> 3 tungsten weights
ST-T2= 4.3oz. Spikes
ST-T3= 5.4oz. Spikes

The spring should simply be the appropriate spring for the tube and buffer length you have - rifle spring for rifle tube, carbine spring for carbine tube. 99% of the time with non-SBR setups you should not get any sort of custom spring with special parameters.

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This is the correct answer

Z-Michigin is spot on.

Buffer and spring length depends on stock type; collapsable (M4-Carbine) or fixed (A1-A2).

Buffer weight depends on how well it ejects. I like the H2 or spikes ST-T2 for most applications.

So if I understand this correctly, the buffer and buffer spring are really dependent on the butt stock.

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They are dependent on the buffer tube, which depends on the buttstock you want to use.

I have a carbine collapsable butt stock with the original buffer and buffer spring. Say I put an A2 butt stock on it - I would want the A2 buffer and buffer spring.Is that right?

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If you change the buffer tube, you MUST have the appropriate buffer and spring for that tube or else things will get damaged. Rifle tube = rifle buffer and spring, carbine tube = carbine (sized) buffer and carbine spring.

A rifle buffer in a carbine tube is far too long and the system won't function - you won't even be able to load it. A carbine buffer in a rifle tube is too short and will result in immediate damage to your gas key, charging handle and the open end of the buffer tube.

the barrel/gas system don't seem to have anything to do with the buffer/ buffer spring.(that doesn't sound exactly right...)

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The basic setup for the buffer/spring is driven by what buffer tube you have, and that in turn is driven by, or controls your choice of, stock.

With a rifle buffer setup there are no further options nor do you need any.

With a carbine buffer setup, IF - IF - you have function issues relating to the gas system being overgassed, THEN you may wish to use heavier buffers. An H buffer will work with most setups, H2 and H3 are not something that will always work with other setups. This is where you get into experimenting.

There are some other specialized systems out there like the VLTOR A5 that have their own rules, but the above covers all current and historical military setups and 99% of civilian setups.

The buffer tube is a separate part from the stock, but for the most part any given stock only works with one type of tube (either rifle or carbine) and not the other.

The rifle recoil system tends to be softer and smoother in operation than the carbine setup. But if you're happy with what you have, keep it. There is also the new VLTOR A5 system which is essentially a modified rifle buffer in a carbine-style tube (3/4" longer than a carbine tube), allowing you to have a telestock with the rifle recoil setup. It also uses a buffer 0.2 oz heavier than a rifle buffer and has options for heavier-still buffers if needed. I've just started using one and so far, so good. You can get the tube, buffer and other parts for about $110 and reuse your carbine stock, although typical carbine stocks will not fully collapse on an A5 tube (only the longest ones - SOPMOD, Emod, STR, ACS - will) but that's not a big issue. FYI, the A5 tube and buffer must be used together, neither rifle or nor carbine buffers work in the A5 tube and vice versa. It does use a standard rifle spring (a $4 part).

Again, if you're happy with what you have, keep it.

The length of the barrel and/or gas system is a separate issue; the only highly relevant issue is the length of the buffer tube.

For AR15s, there are only two (2) lengths; rifle and carbine. That means that there are two types of buffers (rifle and carbine) and two types of springs (rifle and carbine).

You can get lighter or heavier buffers and stiffer or weaker springs, but they all must conform to the basic dimensions of either a rifle or a carbine system.

You didnt really get it friend.

There is no "Carbine tube" "Mid Length Tube" and "Rifle Tube"

There are rifle tubes... which has NOTHING to do with the front of the gun and EVERYTHING to do with the length of the tube.

And "Carbine tubes" which are shorter. Think of "Carbine tubes" as "Adjustable stock tubes" if it makes it easier.



Note the long heavy buffer.



Carbine tube.

You can use a rifle upper on a carbine tube, and a carbine upper on a rifle tube.

The buffer has to match the TUBE... not the front of the gun.

As others have mentioned there are exceptions, but this is the general idea.