Wow, lots post since my last stop in.  I will do my best to answer the questions.
The trunnion and rails were heat treated 36RC if I recal correctly, my heat treater used a vacume chamber and  a "gas" quench...whatever that means.
My buddy used his tig welder and used AWS A5.18 rod.  No chrome lining but the FN print calls for it.  I will never beat it up as much as a mil-spec gun.
My receiver channel was from gunbroker and was just a little "square" on the profile.  The FN prints call for compound radi bends, mine were 90's.
The receiver did warp a litte in at the center because I did not put in a block to keep the minimum rail ID...live and learn.  I will fix that be blocking and bending them back out just a nudge.
I was able to move the bridge forward from the location in the privious pictures and now there is only a very small gap at the rear of the top cover.
Here are some pics of the weld up so far.  Next time trigger group, charging rail and front mount then on to test fire!

That looks awesome!

I looked and may have missed this within the thread, so please forgive me if that's the case.

Could you tell us a little about your sources for parts?  Did this start out as one of those SAW kits I've seen for sale at Knob Creek Machine Gun Shoot, or have you been getting parts a little at a time from different sources?

I bought a incomplete kit, and then parted out the rest over 6 months or so.

I got a little more work done today.  I machined the locking notches, cleaned up the welds and straightened the left rail.  I also set the bridge angle 16 deg. and drilled the holes for the stock.
The lower stock hole has a different location on this design then on the FN model.  The top stock hole was off after drilling, so I went back and remeasured and it looks like the set up moved...I fixed it by using a reamer to open up the
top stock hole and it fits nice and tight now.  Next week I back to the welder to finish the trigger block, ammo pouch mount, forward pintle mount and charging handle.  I will do clean up and then test fire.

thats a sharp looking rig –– nice work on the build!

BADAZZ  

Cheers

I have been watching since day 1. Great work as usual, OP. It is really coming together now.

I am thoroughly impressed man. That is freaking incredible.

i recently got a 1919 kit and someday after i finish that i can only dream of building one of those. how do you make the smiley face with the drool?

out of all the beltfeds i have had the pleasure of playing with (mk19, m2 and 240) the saw is one of my favorites right behind the 19.

Thanks guys.  I was in a heavy weapons platoon while attached to 1/5 at Pendleton back in 93'...don't know if they are still out there and I loved all the big belt feds. I didn't get much time on the MK19 but we did tthousands of rounds on the M2, M249 and M60E3.  Lots of fun shooting on Uncle Sams dime.

Well another day done in the shop and she is all welded up.  I did some final fitting and assembly prior to tommorows test fire at the range...wish me luck!

Wow!  Just wow!  I would love one of those...

Well I'm just back from the range.  It took a few trys but we finally got her running good.  It seems the notch for the ejector spring needed to be cut a little deeper and and adjustment to the spring itself fixed the malfunctions.

So...  how much are you going to charge for these?  

Well it took 6 months to find all the parts and about 60-80 hours in the shop to finish...I can't take credit for the trigger group and bolt/op rod group...that was all designed by Eric Grimes Phd. at US Machingun Armory, his help in the project was priceless.  I think when it was all said and done I have about $6000 into it.  If someone is in the S.WI area an wants help with their own build look me up and we will work something out.

That is amazing.  Good job, and thank you for keep us posted on this project. I can only dream of having the technical skills to do such a project.

HOLY SHIT... I would like to build one someday... GREAT WORK MAN!!!

Wow! That's all I can say. That sir is an awesome project.

what was need to be done to convert it to closed bolt? any pics of that process or just diff parts?

and awsome job, lots o' drool.

Close bolt was a swap of auto parts for semi parts including the trunnion, right rail, and trigger support, op-rod and bolt group.  If you go to the US Machinegun Armory site they have pictures that explain it well.  Basically the barrel trunnion used a press in brass bearing to prevent the installation of a FA op-rod, which also acts as a base for the new action spring which is located in the gas tube between the piston and the trunnion.  The new production trunnions have this feature built in so there is no need to retro-fit that part.  The right side rail is thicker than a FA rail and will only accept the modified bolt/carrier group.  The trigger housing is about 1/4 longer than a FA trigger group so the trigger support will not support or fit the FA trigger group.   The whole system and parts are available through US Machinegun Armory however I pieced mine together from other suppliers only to find out later on some of the parts were not to spec...buy your parts from US MGA!  They are the best hands down and Eric was more than happy to help my with all the questions I had during the build...I even called him the day of the test fire and he told me how to fix the problem with the ejector jumping out and causing a stoppage.  I may have figured it out on my own eventually but it was nice to have an expert on your side.  Shipping was slow at times but the quality of the product was well worth the wait.  If I were going to do it again I would build a welding jig to act as a heat sink and aid in set up, some of the parts twisted and warped during weld up because of the heat applied.  You can do it with clamps and back braces as well but it would be much easier with a jig. Also there needs to be a feature to lock the bolt to the rear during loading and barrel changes.  I think Eric is working on a new charging handle that does that...I will modify my own with a simple spring and pin system suggested by Eric until the new part comes out.  I also ordered a longer barrel and the new style bi-pod, I did not expect the shorty to bark so loud and the old bi-pod gets in the way a lot during carry.  I originally thought I was going to do a traditional set up but the project evolved into the "this works better" idea.  I am very happy with the way it turned out, I was bummed to hear the issue with the registered sear, however that may work out in the end, plus I don't have 17K right now, LOL.

at least their drop in parts. it would probably be a major pain to try to convert from open to closed.

I was lucky I found Us MGA when I did, I was going to try to come up with some crazy sear/striker thing like in a bolt action rifle.  That would never have worked now that I look back on it.  The HK trigger pack and modified hammer seems to be the best way to go to me.  Other people might weigh in though as I only have experience, little as it is, with Mil-spec and the HK system.

Well I finished with the paint and also received my second barrel...I like this one better for balance.  The only other thing I am going to change it the old bi-pod to the new style.  Weight is not a concern for me as I will never take it to the field.  This is the first time I have used the "Norells Moly" product.  It is ok...I am used to a smoother finish like Duracoat this is very slip resistant even when well oiled.

BUMP!

Nice job getting this done!

Way Cool!  Thanks for sharing your M249 project

I'm in love!  

Wow this is awesome.

Can we please see some bump firing?

That is awesome. And, if I may... a grip-pod might be a good toy for this. I had a mk48 for a little bit and I rocked a grip-pod and it was almost perfect. Good for shooting standing, and as long as you don't slam it down hard on the legs it'll work fine.

Looks great!

What with the selector switch?

The trigger housing was milled 3 possition for adding a possible auto-sear at a later date...sadly the housing will only accept milled and welded searless trigger packs at this time due to BATFE requirements

I'm getting a few request for assistance with dimensions for weld-up and parts from my prints so I have decided to convert the critical dimensions and datums to english/jpg format and I will post them up here soon.  There will not be enough info to make your own parts...but with a good calipers you can check if your parts are up to spec.  The only info I will not include is the location of the trigger support and FA sear cut out as I used the SA closed bolt system adn I can not speak to the function of that system. Besides this information is available through other sources such as the Receiver 9348202 prints being sold on GB.  Those prints are useful for several target datums and some weld specs as well.  More to come.

Tell you what, if you can make it to Vegas with that thing, I'll let you shoot a 60mm mortar if you let me shoot that!  Or if you can make it to GA, you can hunt some white tail on my land next year.

Cool, first time I shot a 60mm mortar was in 92'...We(0352's) were guests at a motar shoot and if I recall correctly the section leader had a running bet. Anyone who put their first round on steel at the range, it was littered with old jeeps and dumsters and such, won a case of beer.  Needless to say the guns were already set up and zeroed...but one lucky shot later the beer was cold and free that night, thank you heavy weapons section!

The print calls for 1.75mm +/- 0.03 sheet steel 1015-1018 cold rolled, temper rolled with grain size of 4-8 and the grain running lengthwise. That is about 15 ga. steel.  All the dimensions on the print can run +/- 0.010 and still be fine because the weld up will be set off the center line of the bore as it sits in barrel trunnion.  If you wait to bend the bottom of the ejection port until after the weld up it will be easier to fit a jig into the channel to act as a heat sink and reduce twisting and warping due to the heat from the weld.  I have not included that bend in these drawings to avoid confusion.  BTW I know alot of you have far more experience in machining and welding than I do, so if you see something you think would work better by all means chime in!

this need to be tacked......

The barrel trunnion or barrel support, must fit snug and square in the receiver channel. The trunnion print calls for 30.18 mm between the gas tube and the center of the barrel but the final print calls for 30mm between the two.  About 0.007 Inches diference, so it is within tolerance.

Do you know the difference between USMGA's closed bolt barrel trunion and open bolt barrel trunion? I just bought a receiver channel and front support on GB from the same guy I think you got yours.  The price difference between USMGA's receiver channel was huge and looked identical. Were you happy with yours overall?
Where would be a good place to pick up some blueprints for the M249?
Thanks in advance.

MGA's original  barrel trunnion used a press in bushing to prevent a FA op rod from being installed, the new generation ones have changed the spec on the rear portion of the lower hole to do the same thing without the need for a bushing I think.  I had to do a lot of machine work to get my GB channel finished...but it can be done. MGA's channels are the best I have seen on market to date and they are almost complete.  There were some really good ones out there a few years ago too, but they sold out well before I started my project.   Overall I was very pleased with my channel from GB, the seller has improved his design to include alot of the small holes and cut-outs which makes it even better.  I only ran into an small issue with the rear bridge fitting but I just removed a little metal from the outside radi of the bends for 0.600 up the channel and it fit fine.  But that may have been a measurement issue on my part.  the MGA trigger group fit a little closer to the channel than a mil-spec one...I just adjusted along the way.  You try to hold set-up as close as possible...but there is some wiggle room built into the design.  Most folks selling parts have prints but are not willing to part with them because they think letting out copies would hurt their business or flood the market with junk parts, or it could be the copyright thing too.

just curious, but based on the design you used, Would an AR15 Trigger group work?

Obviously the lower would have to be different, but just looking at it and wondering

Yes, I think an AR15 trigger group may work if you re-designed the hammer like MGA did for the HK trigger pack, although I don't know the geometry issues with an extended AR15 hammer and the clearance with the bolt carrier group. You need about 0.750 extra length to hit a firing pin protruding from the bolt carrier.  If you had a different striker system built into the bolt carrier group then you could use the hammer as-is. My project would not be done now if I had to "re-invent the wheel" so to speak.  Most of the other SA conversions I have seen on the internet also us an HK grip or trigger group.  It just seems to fit the design of the gun. Another advantage to using the MGA design is that it has already been submitted and approved by BATFE firearms technology branch for civilian legal status, SA only of course.  I supose any new or variation of the design would have to be re-submitted for an opinion.

Here are the critical specs on the gas tube support.

http://i861.photobucket.com/albums/ab177/rncbinc/M249SupportGasCylinder9348205.jpg

Good news!  I figured out a legal way to FA this project.  Here is the link:

http://www.machinegunarmory.com/sotnolawletter.html

My friend is a dealer/mfg. and he will register and keep a FA trigger pack in his inventory.  We use my gun as a "host" under his supervision and it never changes permanent status.  Paper work will be done soon and a video posted soon after the test fire!
I think I need to load more belts

For welding up the barrel support and gas tube support,  I just used clamps, but if I were going to build another one, I would make a "T" block that fit into the channel with an insert for the gas tube hole on both ends and bolt that in place to hold everything in alingment and act as a heat-sink during the weld-up.

Here are the measurements for the rail/guide placements.

mcbinc,
Were the rails that you used semi-only rails or full-auto-rails (?)....or is only one set of rails being manufactured (?).
Regards,  RichardS in MI.
US Army, Retired

The left rail is the same SA or FA, the right rail was from MGA and is a little thicker from the top contact surface to the bottom.  0.155 vs 0.125  either way if you measure from the top contact point as shown in the diagram it will be the correct location for both SA and FA right rail.

I forgot to add the  "If I were doing this again"  a large heat sink which would maintain the minimum distance between the rails and keep them from warping is a must It would be a large "T" block with either pins or cut outs to support the rails.  You could also pre-set the 16 deg. angle needed to weld up the rear bridge at the end of the block.

mcbinc,

What was the reason for the inward bend in the Channel at the botom of the ejection-port (?).
Did you have any "issues" with the gas-port-size overcoming the MGA semi-ony system (?).
Great job on your project.

I'm looking at a slightly different way of doing the same thing (but still staying within the semi-only requirements as a rifle).....and really looking hard at the AR-15 trigger-group/parts. There has to be a way to make this do-able as a close bolt system using the AR-15 trigger-group parts.
I like the ideas that MGA has come out with on the permanent extensions for the Para-Barrels.

Regards,  RichardS in MI.
US Army, Retired.