Upper and lower don't fit

[verdugo60]

I have a complete AR Busmaster predator that shoots both heavy and standard projectiles 1/2 moa. I bought a deleware arms lower and my smith installed a lower parts kit and a ctr buttstock on it.

I plan to SBR the new lower for a 300 blackout upper. My concern is that when I take my bushy upper off it's lower and try to put it on the new lower it doesn't fit. Do I need to be concerned? Do I have an out of spec lower, or does bushmaster have a differnt spec for it's rifles? The front pin fits when mating the two, but the new lower seems to collide withe the bushy upper before being able to snap the second pin in. By pushing very hard I can get it close, but will still not fit.

Suggestions?

 

[jasonk]

Re: Upper and lower don't fit

Yes, be concerned. Hard to tell which is giving you the issue, but likely the new lower.

If I was going to spend the time and money to SBR something then I'd plan on owning it forever, so I'd buy a higher quality lower like Noveske, Mega or Seekins.

 

[cwillits505]

Re: Upper and lower don't fit

^^^^^ yep.

 

[Papagallos]

Re: Upper and lower don't fit

Question: Does your new lower sport an aftermarket Magpul BAD Lever on it? If so, that may be the reason it is not mating to the upper. I previously had a BAD Lever on my Stag Lower and the Precision Firearms Upper would not mate to it. It turns out the BAD Lever was in the way. I removed it, and everything fit well.

Hope that you find a solution to your problem. Good luck!!!

 

[VJJPunisher]

Re: Upper and lower don't fit

ran into this alot using RR lowers with other uppers, guarantee ya its the size of the rear "lug", file some off the rearmost corners of the rear lug and bet ya money itll fit and work right fine

 

[bcw1284]

Re: Upper and lower don't fit

also, make sure the hammer is cocked when trying to mate the two halves, they often do not like to close correctly when decocked if the front pin is used to pivot the upper down into position. BTDT.

 

[verdugo60]

Re: Upper and lower don't fit

All good suggestions, thanks guys. I wish it was as simple as making sure it was cocked or a BAD lever but alas, I thinks it's the lower.

May try VJJ's suggestion and shave on the lugs, but I hate to do that with a brand new lower. Would sending it back to them be a smart option? Sounds like a fairly common problem if RRA had the same issues. I might try it on a friends upper and see if it's the same result. The rest of the lower seems tight, this is just TOO tight. Figured I would save a bit on the lower and put it into a Noveske upper once it's SPR'd. Might regret that!

 

[EagleScout501]

Re: Upper and lower don't fit

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: VJJPunisher</div><div class="ubbcode-body">ran into this alot using RR lowers with other uppers, guarantee ya its the size of the rear "lug", file some off the rearmost corners of the rear lug and bet ya money itll fit and work right fine </div></div>

I have seen the exact same thing before, a friend of mine tried using one of my upper on his RR and it didnt fit right, so we tried another upper, and another......we figured out its the lug after we saw the finish being scratched off from trying to put it together.

(let me say though, do not start shaving off metal unless you are sure you know what you are doing)

 

[salmonaxe]

Re: Upper and lower don't fit

RRA Lowers are known to have a high shelf. This is so they can't use a lightning link or RDIAS.

The lower you have could also be made so it doesn't accept the LL or RDIAS.

On my RRA lower there is about 1/6-1/8" of clearance between the upper receiver's rear lug and the shelf of the lower. Some uppers might have long rear lugs.

My RRA lower is compatible with the Vltor MUR upper, so I think it's more of combination of out of spec uppers and lowers.

I agree with jason. Get a good lower if you're going to SBR. I'd probably get an LMT, KAC, AXTS, or Noveske.

AXTS is a really cool new lower that has been really well thought out.

https://axtsweapons.com/products/AX556/release

http://www.militarymorons.com/weapons/ar.uppers4.html

 

[verdugo60]

Re: Upper and lower don't fit

So I took it over to my buddies and we tried his upper on it(another bushmaster.) We started comparing lowers and the only difference is that where the upper's rear lug nut slides into the lower it is too tight. In other words the lower does not have enough machined on it's inside slots to allow the lug to slide in. I am going to try a VTAC upper on it in the next few days. I doubt it will fit which leaves me with two options in my mind:

1: Take a dremmel to the lower to recess enough to allow standard upper lug to fit. This sucks a bit.

2: Contact manufacturer and see if they can do the machining and re-anodize. Sending it off is not a big deal as I only got it for a 300 Blackout upper i don't have yet. That's a big IF they will do anything about it.

 

[BERTMAN77MK2]

Re: Upper and lower don't fit

+1 on jason k info--get a better lower--sell that one!

 

[VJJPunisher]

Re: Upper and lower don't fit

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: verdugo60</div><div class="ubbcode-body">

May try VJJ's suggestion and shave on the lugs, but I hate to do that with a brand new lower. </div></div>

I never said anything about shaving on the LOWER, i said trim the reag lug on the UPPER. for one its easier to do, and two generally speaking a stripped upper is alot easier to replace if need be than a rear lower.

the problem simply put is the rear cutout in the lower is smaller than the rear lug on the upper.

If you want your bushmaster to be able to be used on your lower than that is what you need to do, slowly trim the rear lug of the upper. Now I understand if you dont wanna cut or file on a bushmaster upper that has its own lower, so be it and wait till you get your SBR upper and see if it fits, which i doubt it will so do it to just that upper. I personally say fuck it man they are tools and make em compatible. who gives a shit what the upper rear lug looks like, its not visible when assembled anyway. If you trim slowly you are essentially creating a "custom fit" rear lug.

advising to sell it and buy a "better" lower is absurd in my opinion. Of course I dont fart dollar bills and shit gold bricks either. You already have it, its not hard to make it work.

Push the front pin in shotgunned open style and lower the rear of the upper into the lower slowly and you can find where it contacts and binds and where and how much to file.

I have 2 RR lowers currently, one with an armalite upper receiver and one with White Oak upper reciever, both had to be trimmed to fit in the RR lowers.

below is the white oak upper on my MK12 MOD-0, i left it raw cause i dont give a shit what it looks like, it needed to be filed ever so much on both sides of the lug

next is the Armalite upper that needed the rear bottom cornoers contoured a hair to clear the small RR lowers well when shotgunning it. I spray this one with some black paint for shits and giggles

both guns are sub MOA and the fit is tighter than if they were actually bedded. If you plan on selling the damn thing (which i doubt since you mentioned SBR'ing it) and you have deep ass pockets to go buying another lower, then by all means go for it.

 

[verdugo60]

Re: Upper and lower don't fit

Dude, thanks for the pics and the info, really appreciate it! I am not going to do anything to the bushy upper, since it shoots great and fits fine in it's own lower.

I am also not made of crapping-bricks-o-gold money and don't want to hassle with buying and transferring a premium lower and then swapping all my parts out, for now. The barrell seems to be where you want to spend money for accuracy, so I may just leave the lower and use your suggestion to fit the upper to it when I get it put together.

One other potential problem I noticed though, is that new lower is marked 556. Will that be a problem if I put a 300 blackout upper on it? If I SBR it since I am "manufacturing" it I could just engrave Cal:Multi on it right?

 

[Beef82]

Re: Upper and lower don't fit

I've had a Model1 and BCM upper both stupid hard to get together to pin in RRA lowers, but since you essentially have two decent levers I just forced them, less than a thousand rounds and a few field strippings, they wore in, close up easy now, zero play.

 

[bcw1284]

Re: Upper and lower don't fit

What VJJ said makes sense, I just wanted you to try the easy stuff first.

On a related note, some time ago I got an upper from a hide member here that was an RRA rifle in a past life. He kept the lower so I never really thought about it, but it had some modification done to the rear lug much like what the pictures show. It was a very tight fit to my lower as well, it really took some force to get the pin in but it wasn't unusable like yours. I'm guessing it's an RRA thing with the specs on the lug being different than others.

 

[verdugo60]

Re: Upper and lower don't fit

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bcw1284</div><div class="ubbcode-body">What VJJ said makes sense, I just wanted you to try the easy stuff first.
</div></div>

No worries man, on some days it easily could have been a homer simpson fix...doh! Unfortunatly I'm probably gonna need to buy a steel file!

 

[RufDog]

Re: Upper and lower don't fit

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: verdugo60</div><div class="ubbcode-body">Dude, thanks for the pics and the info, really appreciate it! I am not going to do anything to the bushy upper, since it shoots great and fits fine in it's own lower.

I am also not made of crapping-bricks-o-gold money and don't want to hassle with buying and transferring a premium lower and then swapping all my parts out, for now. The barrell seems to be where you want to spend money for accuracy, so I may just leave the lower and use your suggestion to fit the upper to it when I get it put together.

One other potential problem I noticed though, is that new lower is marked 556. Will that be a problem if I put a 300 blackout upper on it? If I SBR it since I am "manufacturing" it I could just engrave Cal:Multi on it right?

</div></div>

i wouldnt worry about that 556 mark.