XP Action Spring?

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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XP Action Spring?

Postby buckeye3405 » Thu Jan 25, 2018 1:07 pm

I've noticed that when I attempt to load 5 rounds in my Bartz magazines and attempt to charge the weapon the bolt will not close all the way. It seems as though the top round is so tight in the magazine the action spring doesn't have enough "power" to close the bolt. If I load 4, no issue. I'm wondering if an XP Wolfe action spring would help or would this cause other issues? I've had no issues with cycling when firing, however, I've never loaded 5 in a magazine and fired the weapon either.
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Re: XP Action Spring?

Postby Al in Mi » Thu Jan 25, 2018 1:13 pm

Does Bartz use dimples on the edge of the mag to stop the follower, or something up the inside??
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Re: XP Action Spring?

Postby buckeye3405 » Thu Jan 25, 2018 1:52 pm

I'll have to check on that. The mags work fine it just seems as though there is so much spring tension on that top round when loading 5 that the bolt will not close all the way. I have to hit the forward assist a few times to get it into battery. No big deal but if the XP springs will fix this and still cycle reliably I figure why not?
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Re: XP Action Spring?

Postby Bmt85 » Thu Jan 25, 2018 2:24 pm

I would clean up/smooth out the top and bottom of the feed lips and see where that takes you.
Couple of questions, though:
-What style carrier are you using (Auto, Semi, Lightweight, etc.)?
-What weight buffer are you using?
-Is your current spring from a known company, or is it a no name?
-Is the current spring new, or was it used before this build?
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Re: XP Action Spring?

Postby buckeye3405 » Thu Jan 25, 2018 4:24 pm

I used a stripped Anderson lower for the build and have all new lower parts in it. I have a magpul rifle length fixed stock, rifle buffer and rifle spring

It's really not that big of an issue, really. I'll try an XP rifle spring that I have at the house that is NIB or I can load 4, charge the weapon and top the mag off so I have 4+1 rounds at the ready. I've had 0 cycling issues with the rifle while shooting, just seems as though 5 rounds in the mag is awful tight.

My bolt carrier is a standard Bushmaster bolt carrier. I purchased a complete 16" Bushy upper when I did my 450 build last February. Thing sure is a shooter!!
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Re: XP Action Spring?

Postby plant_one » Fri Jan 26, 2018 11:00 am

being a new build - you might need to cycle it some, and do feel free to spray some hornady one shot down the buffer tube to lube things up.

also - are you dropping the bolt with the bolt catch release or are you pulling back on the charging handle?
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Re: XP Action Spring?

Postby buckeye3405 » Fri Jan 26, 2018 11:51 am

I've tried both from a bolt closed and bolt open. The bolt doesn't close into battery either way. I think it's more of a new / tight mag spring than anything else.
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Re: XP Action Spring?

Postby plant_one » Fri Jan 26, 2018 2:14 pm

how new is this upper and lower?


if its basically brand new heres something taht will probably help you.


take it apart and remove ALL the lube from the bcg and upper. you want it sandpaper dry. use something like bore scrubber or brake clean to get it all off.

manually cycle the action 150-200 times or so. no mag or anything. pull the bolt all the way back back and let it drop.

this will wear in the upper and mate the contact surfaces while wearing off any tooling marks. you're essentially speeding up the break in process that will happen during use and lapping everything smooth by just friction.

take it back apart, clean reallllly well, relube, and see if that helps things.



its entirely possible its a mag spring issue, but there could also be some drag somewhere else in there causing the hangups too - even if all your parts are in-spec. a rough spot of tooling chatter on the mating surfaces of the reciprocating parts of the action can cause all kinds of issues, especially those were a small amount of tension makes all the difference between success and failure.
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Re: XP Action Spring?

Postby buckeye3405 » Tue Jan 30, 2018 8:11 am

Put in the Wolfe XP action spring last night. Hit it with a good dose of white lithium grease spray. I loaded up both of my Bartz magazines with 5 rounds and the rifle loaded both from both the closed bolt position and from a bolt lock position without trouble. I need to take it to the range and shoot some 275gr Barnes XPB Black Butterfly ammo i got and see how it cycles. Once again, I've not had any cycling issue with this rifle even with the original action spring.
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