RMW Extreme Build. It has begun.

[Pablo]

Before we do a better consolidation, feeding an AR with a case mouth spacing untapered round can be problematic as well. If the ammo has zero crimp, the gun will choke. Generally a mild crimp will be fine if every area from the magazine to the chamber is deburred and polished.

[mrpipesmkr]

Before we do a better consolidation, feeding an AR with a case mouth spacing untapered round can be problematic as well. If the ammo has zero crimp, the gun will choke. Generally a mild crimp will be fine if every area from the magazine to the chamber is deburred and polished.

Pablo is dead right on this.  I hadn't really noticed the lack of crimp on the factory rounds I have bought. After talking with Pablo I checked out the crimp on some Armscore rounds I had purchased on sale. No real crimp on them at all! Just the bell removed and that's it.
  If you look at a10mm round you can see why this creates a problem. The case mouth on the 10mm is always very far up the bullet, just at the point the ogive is ending. This is the perfect position for the rim to hang up on anything and everything, which it does. Check your rounds out and crimp them if they need it!  It doesn't need to be an exceptionally tight crimp, but it must roll the rim into the bullet at least slightly. It will still headspace properly with a good roll crimp.

[sqlbullet]

Thanks for the feedback.

At some point I wanna build a DI 10mm upper, but it is down the road.

Short term this is relevant to me on my 358 Win build.  I want to see how light I can do it.  Based on mfg. information, I think I can end up under 7lbs on an AR-10 with iron sights.  But, there are two big parts of getting there and one is a JP Rifles low mass BCG for DPMS pattern AR-10.  That bolt is used in conjuction with an adjustable FSB, and probably a Springco Orange spring.  The idea is to use the light bolt, buffer, dial back the gas to get full bolt travel without battering and then let the extra power spring provide the umph to get the next round fed.

I have a good head start in weight using the new TN Arms polymer AR-10 lower, which only weighs 6.8 oz.  And I tracked down a 16" medium contour 358 barrel from KAK.  I may pay to have it re-contoured to more of pencil profile if I just can't get the JP low mass to run.  The idea would be to shave 3-4 oz from the barrel contour to add back into the buffer and BCG.

Cutting to the chase, I am keenly interested in properly understanding the results of changing reciprocating mass components.  I would love to end up with a guide sized gun with 25 rounds of 358 Win on tap as fast as you can squeeze the trigger.  LPK gets here next Thursday.  Handguard is on backorder and that wicked expensive JP BCG has to wait for my 2015 bonus to be paid, probably in March.

[Pablo]

Well. Now I am riveted. 

[mrpipesmkr]

Thanks for the feedback.

At some point I wanna build a DI 10mm upper, but it is down the road.

Short term this is relevant to me on my 358 Win build.  I want to see how light I can do it.  Based on mfg. information, I think I can end up under 7lbs on an AR-10 with iron sights.  But, there are two big parts of getting there and one is a JP Rifles low mass BCG for DPMS pattern AR-10.  That bolt is used in conjuction with an adjustable FSB, and probably a Springco Orange spring.  The idea is to use the light bolt, buffer, dial back the gas to get full bolt travel without battering and then let the extra power spring provide the umph to get the next round fed.

I have a good head start in weight using the new TN Arms polymer AR-10 lower, which only weighs 6.8 oz.  And I tracked down a 16" medium contour 358 barrel from KAK.  I may pay to have it re-contoured to more of pencil profile if I just can't get the JP low mass to run.  The idea would be to shave 3-4 oz from the barrel contour to add back into the buffer and BCG.

Cutting to the chase, I am keenly interested in properly understanding the results of changing reciprocating mass components.  I would love to end up with a guide sized gun with 25 rounds of 358 Win on tap as fast as you can squeeze the trigger.  LPK gets here next Thursday.  Handguard is on backorder and that wicked expensive JP BCG has to wait for my 2015 bonus to be paid, probably in March.

Sqlbullet,
  I've got to say, you are a very ambitious guy to come up with the idea for a rifle like the one you describe!  I carried an M16 around in the military along with some heavier stuff and can appreciate your desire to knock every ounce possible off your build.  It can be done I think, but you are going to have to do a very precise balancing act with your components to make it run. You'll end up with a pile of spare AR parts that were tried but did not work. That's not really s bad thing as you seem to have several other AR's to keep running also.
   I was thinking about your build and my response to your question regarding the H2 buffer and spring.  Weight wise, the H2 is out for your build unless you just have to have the added mass to get your weapon to cycle. 
   However, I wanted to suggest to you that it might be worthwhile to look into the Tubbs flat springs for your build, either the carbine or rifle spring, depending on your build.
   The way the Tubbs springs exert more power with the bolt closed and the way they smoothly stack in recoil would, I believe help your build to be more reliable without adding much, if any, weight.
   Just a thought on my part but check them out.  I really like the way they run in my builds.

[Pablo]

This one should work: http://www.davidtubb.com/tubb-springs/bufferspring-sr-ar10

I'm thinking a 358 Win will have plenty of gas.

[sqlbullet]

Thanks!

I will add it to the list in place of the sprinco orange.  Or maybe in addition.  $30 on this build won't break the bank.

I also came across this the other day:

http://www.vertexops.com/V_Seven_308_AR10_DPMS_SR25_Titanium_BCG_p/v7-bcg-ti-ar10.htm

Same price within a few bucks as the JP low mass, so I am gonna grab one when they are back in stock.

Once I get to the actual build part I will post up a thread on how it goes. Appreciate the input guys!

[Pablo]

WOW that is nice. But expensive!! Titanium is light and strong, but are the thermal properties OK? Probably.

Doesn't  look like AIM makes a lightweight .308 BCG. Darn.

http://www.aimsurplus.com/catalog.aspx?groupid=2535&name=Bolts+%26+Bolt+Carrier+Groups

Why do you need a super light BCG? Are those undergassed? I'm confused, I guess.

[mrpipesmkr]

Thanks!

I will add it to the list in place of the sprinco orange.  Or maybe in addition.  $30 on this build won't break the bank.

I also came across this the other day:

http://www.vertexops.com/V_Seven_308_AR10_DPMS_SR25_Titanium_BCG_p/v7-bcg-ti-ar10.htm

Same price within a few bucks as the JP low mass, so I am gonna grab one when they are back in stock.

Once I get to the actual build part I will post up a thread on how it goes. Appreciate the input guys!

I've got to echo Pablo.  WOW!!  Looks like quality stuff though.   How hard does the .358 feed?  Is it possible to have too light a weight in the BCG?  I suppose a heavier spring or buffer could be used if the BCG is a little light, and still represent a weight savings.
  Don't make book on it, but I believe the thermal properties of titanium are similar to aluminum.

[sqlbullet]

It is nowhere near aluminum, which has great thermal conductivity.  It runs about 25% behind carbon steel, but this won't be a gun that I do mag dumps from. 

With regard to why.  Typically a "light" ar-10 will be 8-9 lbs before optics.  My goal is under 7 lbs to get it competing with a bolt action guide gun.  This BCG will save me 7 oz, nearly half a lb.  I will have to use an adjustable gas block to keep the gun from being ferociously over-gassed, and then ensure I have enough spring to feed a new round.

And it may not work at all.  In which case I will have a very expensive titanium bolt that either sits on the shelf, or that has an insanely heavy buffer behind it.

But, I think I can tune it in just fine.  Lots of guys are making various low-mass BCG's run, so I figure I can find the magic combination.  And if it works like I think it will, I will have a light recoil gun in a heavy hitter cartridge that weighs 8lbs with a full load of 25 rounds.

[Pablo]

It is nowhere near aluminum, which has great thermal conductivity.  It runs about 25% behind carbon steel, but this won't be a gun that I do mag dumps from. 

With regard to why.  Typically a "light" ar-10 will be 8-9 lbs before optics.  My goal is under 7 lbs to get it competing with a bolt action guide gun.  This BCG will save me 7 oz, nearly half a lb.  I will have to use an adjustable gas block to keep the gun from being ferociously over-gassed, and then ensure I have enough spring to feed a new round.

And it may not work at all.  In which case I will have a very expensive titanium bolt that either sits on the shelf, or that has an insanely heavy buffer behind it.

But, I think I can tune it in just fine.  Lots of guys are making various low-mass BCG's run, so I figure I can find the magic combination.  And if it works like I think it will, I will have a light recoil gun in a heavy hitter cartridge that weighs 8lbs with a full load of 25 rounds.

Yes. I know about Ti. We use Ti spacers when we want lightness, strength and LESS thermal transfer.

Your answer makes perfect sense and that is a huge about of mass, gone.

[mrpipesmkr]

Over the past five weeks or so Pablo and I have been in close communication discussing our 10mm and 9x25 carbine builds from RMW extreme.  By explaining to each other improvements we had done to our builds and future plans to improve them we set up a situation where we brainstormed our way into making our carbines run very reliably.  They can be made to be very reliable, and when you reach that point they are a lot of fun to own and shoot.
  This excersize has proven to be a lot of fun for both of us and a learning experiance that is hard to match in terms of pistol caliber DI AR's.   But, we don't believe that every owner of an RMW extreme build would want to go through the many ideas and changes we have had for our builds.
  Our hope has always been to distill what we have done to our builds down to an easy to follow list of important improvements that others could use to improve thier builds.
  So, the following is a distilled list of things we have done. The list is in no particular order but is fairly comprehensive. We will add any other improvements we might have done should we recall them at a future date.

1)    installed Tubbs flat wound spring, Pistol and 300Blackout model.
2)    installed H2 buffer, weight 4.4 ounces
3)    made sure the buffer and spring were lubed
4)    used both the JP and standard rings, depending on build, well broken in
5)    Polished the bolt face until it shined, making sure any ridges were well rounded
6)     polished the cam pin, rails the carrier runs on, hammer face and the ramp the               
        Hammer runs against.
7)     used 600 and 800!grit paper to smooth the chamber walls
      polished the chamber walls including the cone at the start of the chamber
9)      polished the firing pin
10)    Trimmed, rounded and polished the bottom two feeding lugs on the bolt insuring that enough material
         was removed to insure the lugs cleared the magazine. Smooth polish eliminates drag on the
         next round feeding in the magazine


    Both Pablo and I will try to answer any questions you might have. The important thing to remember is that with a little tune up work you can have a very reliable carbine that is tremendous fun to shoot.
   It is important to remember that Pablo's build and my two builds are a little different.  Pablo's build is based on a lower that uses GG magazines. My builds both use the QC10 lower and run Glock magazines.  Still, the work we did on our pieces was very similar with a few small variations.

  I wanted to clarify number 10 on my list. Pablo and I had been discussing the trimming we did on the two bottom feeding lugs of our AR's, even exchanging pictures. He pointed out to me that he had removed material from the rear of both lugs before polishing them, something I had not done.
  The idea here is that by removing the material you lower the height of the lugs and they taper up to full height at the front. This leaves plenty of material at the front to strip rounds from the magazine but allows the rear of the lugs to pass over the round in the magazine with minamal contact until the lug is beyond the crimp. Polishing means that as the two lugs scrape across the top of the magazine round they have very little friction or drag. Because of this, they also tend to move the magazine very little. All this contributes to a much smoother feed, ejection, ect.
  So, to trim your lugs you need to take material off the outside of both lugs, thinning them enough that they clear both sides of the magazine and lower the rear of the lugs so that they taper up to the front, which is left full height. Polishing all contacting surfaces insures minamal drag.
  I left a little height on the rear of my lugs so that they will somewhat lock into the barrel extension lugs, but I don't believe this is a huge consideration because the lock, designed for a .556 round, is more than strong enough to handle the 10mm.

[Pablo]

I keep thinking I'll have time to add detail and I just can't catch a break.

[mrpipesmkr]

I keep thinking I'll have time to add detail and I just can't catch a break.

Wow Pablo, you seem to be a busy guy! That's why I updated the list. But, the board will be here next time you have some time.

[mrpipesmkr]

I keep thinking I'll have time to add detail and I just can't catch a break.

Pablo,
  Now that you have your 10mm running so well are you still looking at doing the 9x25?.
  I've got to say that the 9x25 is a great intermediate carbine round.  More versatile than the 10mm with almost as much close range stopping power and better gas pressure to run the AR. They are both a hoot to shoot!