Bolt Carrier has limited travel

[michaelmew]

I was swapping out some parts between my lowers this morning. Got all done and did a function test and found that my bolt would only travel about 1" rearward. I changed from an Ace Skeleton stock to a collapsible. But, I kept the std (as far as I know) buffer and spring. That would not allow the bolt to move back. I had access to a friend's rifle with a Spike's buffer and spring. I put those in my rifle and it functioned just fine. I also put my buffer and spring in the friend's rifle (whihc also has a collapsible stock) and it would not travel more than 1". All signs point to an issue with the buffer and/or spring.

The weird thing is though that with the Ace stock, everything performed flawlessly. I guess I'll be ordering an ST-2 or 3 and maybe a Wolff spring (if you think it is necessary). In the meantime, is this a precursor for other cycling issues that I should keep mindful of or anything else like that? Am I missing something?

-mm

[Drifter]

Ace skeleton stock uses rifle-length buffer and tube. Collapsible stock uses a shorter carbine buffer and tube. If you're trying to use the rifle-length buffer in a carbine tube, it will not allow sufficient travel of the bolt carrier.

You'll need a carbine buffer and spring if you plan to stick with your change to a collapsible stock.

This image shows the buffer differences (carbine on top and rifle on bottom):



Link to source: http://www.gundigest.com/rifle-reviews-a...il-springs

[michaelmew]

O I C, thanks Drifter.

Feel kind of stupid now. Hopefully I'm not the only one that made a rookie mistake like this before.

-mm

[Drifter]

Don't sweat it. We're all learning every day.

[michaelmew]

I want to keep the collapsible stock, so I need to get a carbine buffer and spring for my 18" Grendel. There were some good points on another thread, but I can't remember the concensus. Spikes's ST2 or ST3 or something else? Any need to get a Wolf spring instead of standard? I've got to buy a new either way.

I guess the safest thing is to try my friend's ST-2 and see how it performs?

-mm

[ericv]

Hey, if we all knew it all we wouldn't have this great website!!!

[Drifter]

I want to keep the collapsible stock, so I need to get a carbine buffer and spring for my 18" Grendel. There were some good points on another thread, but I can't remember the concensus. Spikes's ST2 or ST3 or something else? Any need to get a Wolf spring instead of standard? I've got to buy a new either way.

I guess the safest thing is to try my friend's ST-2 and see how it performs?

-mm

Lots of variables, such as gas port size in the barrel, bolt carrier style (M16 is heavier than AR-15), ammo being used, etc. Generally speaking, 18" barrels with midlength gas ports tend to be somewhat overgassed. If you don't intend to use an adjustable gas block, an M16 bolt carrier with an H3 buffer (~5.3 to 5.4 oz) often makes a good combination. I usually use a Tubb flat-wire buffer spring, but don't consider it a necessity.

Using available parts from your own and your friend's rifle to experiment is certainly worthwhile to determine what works best in your setup. And while not everyone subscribes to the ejection pattern theory, spent brass landing somewhere around 4:00 can be an indicator of a system that is in tune. If brass lands forward of that, it can mean excessive gas exists (brass hitting the shell deflector overly hard and bouncing forward), so a heavier bolt carrier, a heavier buffer, and / or a longer or stiffer buffer spring might be needed for optimal results. On the flip side, you want reliable function, and for the bolt to lock back on the last round fired from the magazine. If this isn't happening, go in the opposite direction with lighter components.

An adjustable gas block can often solve the tuning aspect without having to purchase different components.