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First let me say I enjoy watching what goes on within this site. I see a lot of very competent people discussing and helping each other towards a common goal focused around a love for 6.5 Grendel. Now I need some help or clarification from this community. I have only been dealing with AR's for a little over a year. I have developed a fast fondness of this weapon. I am a tinkerer by nature. I never can leave anything alone long. I seem to have always been a Tim Allen kinda guy, Grrr more power.
Now to the point, I purchased one of the Lilja barrels in the A24 style. I had originally purchased a Vltor MUR-1 upper that needed a barrel. I was going to put the Lilja on that but listening to comments made felt that a heavier upper was in order for this barrel so I purchased a Mega billet to use. I didn't know that Mega machines their upper to be heat/chill fit to the barrel. I always put things together multiple times test fitting everything till I get it right. Might say I have a little OCD issue.
This is my problem, I chilled my barrel in my freezer which I keep at -10 (I'm an HVAC Contractor and do it because I can. :o ) When I tried to put it on the Mega upper it would only start maybe by about a 1/16 of an inch. If I heat the upper they will probably mate up but how will I ever get them apart.? I'm used to press fits but that's usually with steel parts that can take the force of pressing them on or off without damage. I'm concerned with this aluminum/steel situation though. As I have only changed barrels on my other factory built AR's with their much less precision fit. I'm apprehensive with this my first scratch build. Please anyone want to jump in on this and fill me in. Is there some trick I need to know, am I being overly cautious?
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It is hard for me to determine how tight your upper to barrel fit is with out being there myself. Most upper to barrel fits are too loose rather than too tight in my experience. I have always been able to wiggle even blue loctite barrels out of the uppers after applying heat. I would be very careful not to tap the barrel in if it is too tight causing a failure of your upper receiver barrel channel. I would measure the inside diameter of your upper barrel receiver and compare it to the outside diameter of your barrel extension to see what were working with.
customcutter
Unregistered
I would rather have a tight upper like yours than one that is loose. But, your problem is wanting to disassemble. If/when you heat the upper, don't use anything hotter than a heat gun. No open flames our you could damage the heat treating on the upper. I could be wrong, but I think you'll find more thermal expansion by heating the upper, than shrinkage by cooling the barrel.
Edit:
You might want to measure the barrel extension and upper too make sure that both are within spec. It is possible to be in spec and be this tight, .0003" to be exact.
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Welcome aboard.
You might consider trying the fit of the barrel in your Vltor MUR upper receiver. That's a good quality upper with plenty of strength.
Drifter
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haustinv Wrote:I didn't know that Mega machines their upper to be heat/chill fit to the barrel. I always put things together multiple times test fitting everything till I get it right. Might say I have a little OCD issue. This is my problem, I chilled my barrel in my freezer which I keep at -10 (I'm an HVAC Contractor and do it because I can. :o ) When I tried to put it on the Mega upper it would only start maybe by about a 1/16 of an inch. If I heat the upper they will probably mate up but how will I ever get them apart.? I'm used to press fits but that's usually with steel parts that can take the force of pressing them on or off without damage. I'm concerned with this aluminum/steel situation though. As I have only changed barrels on my other factory built AR's with their much less precision fit. I'm apprehensive with this my first scratch build. Please anyone want to jump in on this and fill me in. Is there some trick I need to know, am I being overly cautious?
I have used numerous MEGA SBU's and the fit is special I would say. All I do is use my heat gun, for shrink tube but gets to 500 degrees. You just move it around the front of the upper for about 5 minutes not directing the heat in one area. The Aluminum heats up, expands and then just insert the barrel (no freezer required). Before it cools, put the barrel nut on and tighten so it draws it together. Once it cools it is locked tight. If you remove the barrel nut you wont be able to pull it apart. You will need to heat again in the same fashion but use a dowel or perfectly round piece of aluminum rod to help tap it out once heated. It does work, I have removed them using this block that fits in the upper receiver.
http://www.midwayusa.com/product/819480/...uctFinding
Anyhow, once together you might not ever need to remove it. Plan ahead...
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I recently put together a Lilja barrel on a mega side charging upper and only used a hair drier and tapped it in. Aluminum will expand at a much faster rate than steel, I think maybe 4 to 5 times as fast. So you can remove it the same way, I did.
Those megas are tight!
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Before you put it together I would use the upper receiver lapping tool. Look at LRRPF accuracy thread.
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tango29 Wrote:Correct me if I am wrong hoard. I did not think you needed to lap a billet receiver.
Perhaps it's less likely needed with billet, but still sometimes beneficial.
Drifter
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tango29 Wrote:Correct me if I am wrong hoard. I did not think you needed to lap a billet receiver.
I lapped mine but I didn't need to, it doesn't hurt for peace of mind though.
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Thanks for the replies everyone. I found a wood dowel that fit the carrier tunnel in the receiver and got brave with the heat gun and I was able to seat the barrel and then remove it.
Now you've made me stop and rethink part of my plans. I have the JP captured buffer spring, JP full mass ss carrier, POF carrier roller key and a NiB bolt. I was going to have my upper and charging handle NiB'd by WMD. I want this puppy to run smooth as silk. When I got the barrel seated I couldn't see any light at the seam of the receiver and barrel extension flange using a magnifying glass and a very bright back light. You have gotten my OCD tendencies kicked in and I'm probably going to have to lap the receiver now or go crazy thinking about it. So do I lap then coat or coat and lap when it comes back? Also should I not have them coat the inside of the receiver nose where the barrel fits as it's pretty tight already? I measured the barrel extension and receiver as best as possible and came up with a .0025 inch interference fit. Not sure if that's the real number as I don't have a dial bore gauge to get a true I.D. on the receiver.
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Welcome.
If you have seated a Lilja in a Mega I'm thinking you want to leave it alone. Others may disagree but the need for lapping a Mega billet receiver; a precision part, is going to be pretty unlikely. Especially when you consider what repeated heating and hammering of that receiver tunnel is going to be doing?
