using a RRA used bcg in a new LMT upper (new guy)

AR/M4 AR General Discussion
1

I just purchased a new LMT upper. Ive got a slightly used RRA bcg with about 500 rounds through it. I purchased a CMT bolt new and was going to use it as a back up. Can i use the CMT bolt with the RRA carrier or just keep the whole RRA bcg in the upper. Also when i put the RRA bcg in the LMT upper for the first time and cycled it a few times there was a burning smell in the upper receiver. Is this normal or what?:confused:

2

A bolt with only 500 rounds on it is practically new and should headspace fine with a new barrel. Gauge it to be sure if you decide to go this route.

Personally, I’d drop the CMT bolt in the carrier you have on hand and hold onto the RRA bolt as a backup. Some of the CMT bolts are MP tested, I think (not sure about HP). If your’s isn’t, then the only real gain I think you’d be getting is a new bolt mated up with a new barrel.

Bring your carrier staking up to snuff as per the threads in the tech section and run it hard. Also check out some of the info regarding extractor inserts/springs/o-rings, etc for your bolts and address any issues as indicated. None of these (aside from checking headspace) are any reason at all not to shoot it in the meantime - but if you run into issues or plan to run it as a go-to rifle, then they’re worth consideration.

If that “burning” smell is the gunpowder/sulfur scent when cycling, that’s normal. I think it’s a pheromone rifles put off to let you know they’re in season and want to be shot… :wink:

3

I know you guys talk about gauging and head space but is there a link or can you briefly describe what it means to gauge my bolt.

4

The CMT bolt in the RRA carrier should be good to go.I’d make sure the gas key is properly staked and that the extractor spring and buffer are of correct grade.Other than that I’d check the headspace on the bolt and go.
For a great deal of information on headspace,just do a search.You will find more info on the subject than you will want to know.
The buring smell your getting is from the freshly anodized parts rubbing together as you cycle the action.Its normal,some do it worse than others.Smells kinda like wet matches to me.

5

Basically, you need to find an armorer/smith/friend who has a “field” gauge for a 5.56, or pony up a few bucks and order one (not a bad thing to have on hand - you can get them from $10-$30 from most gun parts dealers online).

It’s a simple process of inserting the gauge into the chamber and seeing if the bolt will close and lock (with extractor/ejector removed). If it does, there is too much headspace and you could experience catastrophic failure when firing the rifle (loud boom, lots of smoke, damaged rifle, missing digits, etc). If it stops short of the bolt locking, then you are good to go.

Here’s one at Brownell’s, but I believe you can find them a lot cheaper online elsewhere:

http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=19183&title=FORSTER%20HEADSPACE%20GAUGES

6

Make sure you read a thread if you decide to check the headspace yourself. You need to remove the ejector and slide the bolt/carrier closed “gently”. You don’t just drop the headspace gauge in the rifle and release the charging handle…

7

I personally would run the new BCG in the new upper. Just make sure it is staked properly and it should be GTG.

I would save the used one for a spare.

8

Actually that is incorrect. You would need to remove the extractor not the ejector. If you are using a .MIL gage such as the one that Bill Ricca was selling then there is no need as it has no rim on the gage.