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Posted: 2/24/2010 9:04:25 PM EDT
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Hi
I just built my first AR. Got it all together earlier today and took it to the range. At the range, i was having huge issues with sitght in. To even hit the standard GI targets i was using, i had to adjust my windage fully left, aim to the far left side of the target, and even then i would hit outside the outer ring on the right side of the target. I dont know whats going on here. Im pretty sure ive got this all together correctly. The gas block is exactly where it needs to be, its an MI same-plane gas block, with magpul MBUS sights mounted. When i mounted the gas block, i made sure (with a level) that it was level with the upper reciever. Any ideas? Thanks in advance, Sam |
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Lots of folks on here have had the same problem. Basically it comes down to this:
1. The front sight is canted or loose if rail-mounted 2. The rear sight is off center or loose if rail-mounted 3. The barrel is canted - the barrel locating pin is not in the same axis as the front sight block 4. The barrel is crooked on the receiver I know lots of folks go with flat top rifles so they can conveniently mount optics, and initially run backup iron sights. This introduces more places where manufacturing tolerances can "stack up" against you - the rail can be made a little off, and the sight (front, rear, or both) can be a little off, and if they're all off in the same direction you're going to have rifle that shoots way off. In my experience this seems to be less of a problem with A2's, as you're not matching up parts made by different vendors and expecting precision. In my experience, there's a basic issue with the manufacturing of the upper receivers. If you look at an upper of any type, you will observe that the "bore" through the upper where the barrel sockets in and the portion where the bolt carrier rides, this is made with one boring operation to the same diameter at one time. Then they use a piloted die to roll the threads where the barrel nut threads on. I've seen more than one receiver where the front surface of this "fitting" is no longer square to the axis of the bore through the upper. It appears to me that when the threading die "bites" into the aluminum of the barrel socket, it pulls some of the (soft) aluminum forward, which produces this non-square front to the receiver. Now, when you install the barrel, it's pointed off at some angle. I ALWAYS use one of those Brownell's receiver lapping tools now to make sure the barrel socket is square to the axis of the receiver. My son built a 16" flat-top kit gun, and when we sighted it in it was 28 MOA off to the right and low. We folled around with it, trying to cant the front sight to bring it on target, and eventually just bought a new upper receiver and swapped all the parts from the old receiver to the new one. Before mounting the barrel we lapped the upper - and it WAS off of square - and when we then sighted it in it was almost dead on for windage. He was also running a DPMS BUIS, and honestly I'm not impressed with anything I've seen made by DPMS, so the irons may have been part of the problem, too. |
| Nothing wrong with making sure that the barrel socket is square with the bore line with the tool, but where we differ is if you find this not to be the case (socket cock-eyed), then instead of removing the anodizing coating and making the face of the socket soft by lapping away the anodizing, send the upper back to be replaced instead (read it's out of spec from the start). |
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Does the anodizing really contribute anything to the "hardness" of the aluminum? If so, it can't be much, as it abrades off pretty easily - look at the rear of the upper under the charging handle, for instance. Personally I think the anodizing is more of a corrosion-resisting finish than a surface-hardness treatment, but I may be mistaken.
However, I'm a BIG believer in using the lapping tool to "square" the front of the receiver. As it happens I'm trying to get into the NRA High Power Rifle game, but as my last name isn't Trump I'm trying to do it on a budget. I have a basic Del-Ton 20" A2 kit rifle on a Doublestar lower that I am slowly upgrading as needed. First, this rifle, as it came right out of the box, needed 8 clicks of windage to zero - that's about 4 MOA with the standard A2 rear sight. Also my experience at shooting this rifle at 200 yards from a sling-supported sitting position revealed that my POI would move all over the place with varying sling tension (I'm using a Turner Saddlery leather 1907 sling). I decided that I needed a free-float handguard, which is a fairly inexpensive upgrade - about $125.00 or so - so I ordered an RRA CMP legal free-float handguard kit. I installed it last Saturday. As I had to exchange the barrel nut with the one from the free-float kit, I decided, while I had the rifle dismantled to that extent, I would just go ahead and use the lapping tool on the receiver. As it happens it didn't really need much - just a few hand-powered turns and I could see that the tool was making good contact all around the "ring" on the front of the receiver. I then reassembled the rifle with the free-float handguard, torquing the barrel nut to 35 lb-ft then turning it so the next gap lined up with the gas tube aperture. Today I shot the rifle for the first time since the upgrade. As we have a couple of inches of new snow, and I didn't feel like driving all the way to the range and dealing with the long gravel road there, I just took the rifle out in my back yard where I can shoot at 25 yards down into a dry creek bank. Using Federal XM193 from an unsupported standing position, with my sights set at "mechanical zero", I was pretty much dead on! Of course I'll have to fine-tune the sight adjustments when I get to the range, but going from 8 clicks to 0 clicks of windage is a big plus. |
| Yes it makes a huge difference in the hardness. If you've ever seen anyone that decided to "polish" the feed ramp on their aluminum framed 1911, which removes the anodize, and then looked at how chewed up it gets from bullets slamming into it, you'll understand. The anondize effect will remain even after a scuff or mark that removes the color (dye) but it will not remain when faced with sandpaper, blasting, or a lapping tool. |
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Quoted:
Yes it makes a huge difference in the hardness. If you've ever seen anyone that decided to "polish" the feed ramp on their aluminum framed 1911, which removes the anodize, and then looked at how chewed up it gets from bullets slamming into it, you'll understand. The anondize effect will remain even after a scuff or mark that removes the color (dye) but it will not remain when faced with sandpaper, blasting, or a lapping tool. But even so, considering that we are talking about just the front "ring" of the fitting on the receiver where the barrel nut holds the shoulder of the barrel extension against the receiver.........exactly how much wear are you ever going to see here? Wear infers movement of one part against another. So you swap a barrel (And how many times will you do this?) and torque the barrel nut and you're done. No movement, no more wear! |
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I don't disagree and don't know that it will ever cause an issue... The question was does it make a real difference in hardness and the answer is yes...
In this particular case again, dunno if it will ever cause an issue or not... Just what the technical answer is... |
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Bare aluminum has a surface hardness factor of around 15 to 18 in an untreated or raw state.
When anodized, the surface hardness becomes around a RC rating of 62 (.002 to .006 thickness that has been grown onto the bare aluminum). Between the two, there is a huge difference. Also, you have to remember that anodizing is clear, and pigments are introduced into the pours of the skin before then are sealed by heat to create the colored part. In some cases where you may think that the anodizing has been totally worn away, it's only the very outer layer of the skin that has been opened up and the pigment allows to leach out, still leaving the base of the anodizing skin still in play. |
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Quoted:
Bare aluminum has a surface hardness factor of around 15 to 18 in an untreated or raw state. When anodized, the surface hardness becomes around a RC rating of 62 (.002 to .006 thickness that has been grown onto the bare aluminum). Between the two, there is a huge difference. Also, you have to remember that anodizing is clear, and pigments are introduced into the pours of the skin before then are sealed by heat to create the colored part. In some cases where you may think that the anodizing has been totally worn away, it's only the very outer layer of the skin that has been opened up and the pigment allows to leach out, still leaving the base of the anodizing skin still in play. Good discussion we've got going here......although I think we hijacked the OP's thread. Sorry. But anyway, on the two receivers on which I've used this lapping tool, the first one started to show the effects around the 2-o'clock position. I kept working it until I could see that the tool was making good contact all the way around to about the 10-o'clock or 11-o'clock position. On my rifle which I just did, I could see that I was getting some contact a good part of the way around - say, from 1-o'clock to 5-o'clock, with lighter contact the rest of the way around. I just kept working the tool until I got what looked like good contact pretty much all the way around. It's really kind of a jucgement thing, as the front of the "ring" is not perfectly flat, but has a kind of rolled edge. The tool will start making contact at just a thin line, which, if you keep working the tool, will eventually grow larger. My goal is to move absolutely the minimum amount of material in using this tool. Just enough to make me think that the shoulder on the barrel extension has a good surface to butt up against, and a surface that is perfectly square relative to the axis of the bore through the receiver. The good thing about this tool and its use on AR's is that it has no affect on headspace, as the bolt locks into the barrel extension. Besides, I bet you would need a micrometer to measure the amount of material removed. I did do a little research on anodizing, and you guys are right in that it does add a hardened coating to the aluminum. But I bet that proper use of this tool doesn't cut all the way through this surface anyway. |
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Depends on how much anodizing you are using, and if the tool being spun in the receiver bore is staying true as well with the receiver center line.
Since access to a machine shop, let me put it this way (small shop at the house and large one at the company). I have a upper receiver bar that I can insert into the receiver with wedge locks on it so I know that only is the bar is true in the lathe and the receiver bore line is true to the bar as well. Once in the machine, I can dial indicate not only run out of the face of the barrel receiver socket, but the socket channel walls as well. Depending on the application, if needed, I could square everything, but comes the rub of removing the anodizing, and may have the raw aluminum smear if too much pressure is applied. So lets start with the barrel socket to begin with. If you socket is not true, and you do square the face, then if you have enough slop between the socket and extension, then yes you could true up the face of the socket this way, but when you go to tighten the barrel nut, the greater bearing surface of the extension to the inner socket channel is going to take over, and you will just peen the face of the socket as you go to tighten up the barrel nut . On the same token, if the socket is true, and the face not, that with a tight barrel extension to the socket mate, when the barrel nut is tightened correctly, it will peen the underlining non anodized aluminum to square the face of the socket via the correct three time tightening/loosing before obtaining +35 ftlb/indexing of the barrel nut (read the procedure to install a barrel is not only to mate the threads, but the barrel extension to the socket as well). Don't get me wrong, it's just that I don't think the tool is the best way to Square a barrel socket (channel or face). Had the tool be designed to lock into the upper receiver channel, then the end portion the part that was guided and spun alone, then it may have better merit somewhat, but again your going back to how much of the anodizing and enlargement of the barrel socket would be required to achieve such perfection. When truing a steel bolt action receiver, you are not only squaring the face of the receiver, but also the threads as well. In regards to the threads (would be upper barrel socket in this case), when the new barrel is threaded, then are done over size to match the new deeper threads. In the case of the AR upper, unless you are making a over size barrel extension to mate with the now over sized socket channel if you have to true such, you have just increased the slop between the two, and that is very, very less than ideal. Bottom line here, if you have a upper receiver that is cat'y wompus/ not square, then send it back for replacement. Lapping/machining on any receiver is going to void the manufacturers warranty, and if you run into a problem down the road, your will be paying for a new receiver or the repair costs. If you want to buy the tool to check alignment, that is fine, but really, if you just must wrench on and true the receiver, there are better/more precise ways. |
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