AR Sponsor
Posted: 8/3/2007 12:04:41 PM EDT
|
I am trying to get a good idea of the realistic specs that magazine well is manufactured to across different mfg and time. I need some help from the various lowers. The most important dimensions for fit that I'm concerned with are the overall length and width of the contact surfaces. I only have access to a small number of guns and I have input the info into excell for data analysis. With my sample of only 15 guns, I have the following info. Average Length = 2.409" Width = 0.905 Max Length = 2.42 Width = 0.908 Min Length = 2.4 Width = 0.898 I came across a lower or two that I didn't measure that were way out of wack with regards to the length (This is the more important dimension for me) If you could please take a measurement and post it along with the manufacturer and about the time that it was made it would help me out greatly. Thanks in advance, I will compile data if you post it. |
|
. The well machining dimensions and tolerances are on a MilSpec drawing of the lower. Those drawings used to be available for download from arfcom somewhere. But, I cannot find them now. OK, found it. On the openig page to this site, go down to the bottom of the left panel, then click on Manuals (PDF). Then on the linked page go all the way down to the bottom and you will find a downloadable lower drawing (1575KB) PDF. That should have the dims you are looking for. Actually midway down on the left panel on the opening page under "manuals and documents" |
|
I have seen the prints mentioned. The Values are W=.898 +-(.01) L=2.398 +-(?) I don't know how to interpret the tolerance because it doesn't seem to be directly written for this value. Maybe someone with some GD&T knowledge can help there. What I am wondering is the statistical range of actual lower values, particularly (L),with respect to the manufacturer. |
|
Both dimensions ( 0.898" and 2.398") carry the +/- 0.015" tolerance according to the default title block tolerance shown on the drawing. The individual drawing sections do not show the default tolerance. It is only shown in the title block. The tolerance stack up for the 0.898 dim. is also double verified by stacking up the dimensions 0.500 +000/-004; and twice 0.199 +/-.006 in another section view of the well. Whether broached or milled, the mag. well finished dimensions in the cross section would probably become smaller in succeeding parts, as the tooling wears. The drawing is not clear about the surface build-up and loss of clearance due to the hard coat anodize of the lower receiver, or if the 0.898 and 2.398 are intended to be taken over the hard coat anodize. This could reduce the cross section dimensions another 0.001" to 0.002". Anyway, based on the drawing, given a specific magazine, one lower could be tight, and another one lose, and both be legal within the 0.030 allowed range. |
|
Here are my attempts at the measurements you requested. This is from a mitutoyo dial calipers and from two lowers. Bushmaster: Bottom 0.898 front 0.898 rear Bottom 2.402 left 2.408 right Top 2.398 left 2.390 right Colt: Bottom 0.892 front 0.892 rear Bottom 2.392 left 2.393 right Top 2.394 left 2.392 right From a GDT point of view, the +/- O.015 should encompass all surface variations including straightness, flatness, parallelity, etc. By the way, the requirement to "drop free" is probably not a government specification. Otherwise why would one see pull loops attached to the bottom of magazines as a thrid party add-on? However, there is a significant purpose of the "drop free" function during a tactical reload. |
Those loops are for pulling magazines out of magazine pouches, not out of magazine wells. They also serve as protective bumpers when someone does allow a magazine to drop free from a magazine well. |
That makes sense. Thanks.. Now if we could just get our hands on those "proprietary" USGI TDP's and QAP's for magazines and determine what the acceptable magazine dimensions and tolerances are, that would help get to the bottom of the mag/mag well fit up issue. I have only run into dead ends trying to obtain shop level detailed drawings of finished USGI mag bodies. One would think after 40 years that would be public domain. 
|
|
Now if we could just get our hands on those "proprietary" USGI TDP's and QAP's for magazines and determine what the acceptable magazine dimensions and tolerances are, that would help get to the bottom of the mag/mag well fit up issue. I have only run into dead ends trying to obtain shop level detailed drawings of finished USGI mag bodies. One would think after 40 years that would be public domain. I really wish I could give you those dimensions. No manufacturer can. The only thing I can say about the mag/mag well issue is that it will always be an issue. Our gaging tool is a Colt mag well duplicate and we still have magazines come back as they will not drop free from some magwells. We even made mags special for one manufacturer because his mag well was so small, within tolerance, but barely. Larry C Products |
This is ar15.com. There are thousands, if not tens of thousands, of AR15 shooters. Many have had it with the mag. issue. If they ever get together and develop and publish for public domain a set of "USGI-like" dimensions and tolerances that you must meet to sell your product (visa-vis a industry developed trade spec like an AR-15 ASTM standard), your "No manufacturer can..." will melt away. The story about "custom-slim" mags for a lower mfgr that makes mag. wells too tight for normal mags is revealing. I was hoping this thread would result in a good collection of dimensions for mag-wells which would be a start. Apparently, most shooters don't care about these silly dimension things though. They just want stuff that works. PS: I really appreciate your willingness to air your industry laundry here. While it may be troublesome at times, I am sure it has won C-Products a loyal following, and it has given you a feed-back mechanism that you otherwise would not have. I think it probably goes without saying, you just have to keep collecting data and pounding on your manufacturing and quality control department. |
|
PS: I really appreciate your willingness to air your industry laundry here. While it may be troublesome at times, I am sure it has won C-Products a loyal following, and it has given you a feed-back mechanism that you otherwise would not have. I think it probably goes without saying, you just have to keep collecting data and pounding on your manufacturing and quality control department. Thanks. Yes, the feedback I get from AR15 has gone a very long ways in helping us develop a marketing as well QC strategy. I will continue to post honest answers to questions I can answer. I believe the mag well issue is not a magazine manufacturers issue, but more the rifle amnufacturers. If all mag wells were dimensioned the same, than we could eliminate this problem. All of the manufacturers of .223 magazines, manufacture to the same set of prints. It would be interesting to hear from some manufacturers of rifles if the mag wells are all dimensioned the same? Larry C Products |
AR Sponsor