[Headspacing & Bolt Measurements - check my math]

Tagging to add to my "followed content" list.

[Headspacing & Bolt Measurements - check my math]

I understand what you are attempting to do, but I'm telling you that it's not an accurate way to do it.

You are taking measurements that ASSUME the planes are all straight/parallel.

Even for comparisons sake, it's not nearly as accurate as it could be.

If you want to do it without buying a barrel extension, at least use a surface plate and some 1-2-3 blocks to setup a measuring method that directly measures from the underside of the lug to the bolt face.

Thanks for the feedback. Again, I'm not measuring headspace - just looking at the difference between bolts and the potential effect on headspace.

I figured that if the lug measurement would show how far the bolt extended into the extension, and what I called the "plunge" is how far back the actual breechface is.

Lug depth - plunge + distance off the barrel extension lugs.

[Headspacing & Bolt Measurements - check my math]

You are doing the measurements wrong.

Get a barrel extension and hang the bolts in the extension.

Measure down from the extension face to the boltface.

Here is the block I made that makes it easier:

The nominal dimension is 0.750" and here is how the parts all fit together.

There is another standard depth at 0.763" that is used for the AK based cartridges to increase the firing pin protrusion.

This non-standard spec first showed up from Alexander Arms on the 50beowulf, 6.5grendel and 5.45x39 barrels.

I believe it was because they were converting standard 5.56 bolts and wanted to skim both the boltface and the counterbore in one setup.

Other makers of 5.45 bolts have also adopted this non-standard dimesion as it gives extra firing pin protrusion which helps ignite berdan primers.

[09 MGM Ironman]

Any updates on Squadding?

The date on the mgm site still shows April 9th.

I just want to know if more CA guys have signed up so I can send in my squadding request.

Randall Rausch

www.ar15barrels.com

[HK AR15 Magazines and Lightened Bolt]

The gun worked with USGI mags ... I switch to HK mags and it starts to malf ... I switch back to the USGI mags and the gun works ... Wouldn't it be safe to say the mags are the problem?

Sounds like the USGI mags are smoother.

If the HK mags were new and did not feed as smoothly as your well worn USGI mags, this problem would pop-up as the gun is probably running on the minimal edge of function.

[JP ulmos maintenance]

Physics majors get your pencils out!

I am interested in knowing the total weight of the ULMOS recoil system. Bolt carrier assembly + Buffer.

Stock Dpms BCAs are about 11.2 OZ. I am running the YM Nat'l Match which is 11.4OZ

Stock rifle buffers are 5.1OZ + 11.2= 16.3OZ normally.

My Delrin rifle buffer is 1.8OZ with BT's bumper

My Delrin carbine buffer is .8 OZ " "

I have a plug that I can run the carbine buffer in the rifle tube.

So I can go as low as 12.2OZ total which is 75% of normal mass.

The question is what will happen running the lighter buffers????

Will the buffer spring be able to slow down the BCA better or will it slam harder into the end of the buffer requiring use of an XP spring or less gas?

Will the BCA open sooner when there is higher pressure at the chamber still and affect things?

I played around with this a while back and what I found was that you actually need SOME weights that move in the buffer.

Without any weights, I had bolt-bounce problems in fast double taps.

Those moving weights are there to keep the carrier from bouncing off the back of the barrel extension.

JP uses an A2 buffer shell, but puts aluminum weights in it.

I have been taking A2 buffers and removing 3 of the steel weights.

Then I make a delrin spacer that accounts for the missing weights and the aluminum spacer that I remove.

This gives me a buffer that's slightly lighter than a standard carbine buffer, with just enough moving weights to eliminate the bolt-bounce issue.

For those not aware of what's inside your buffers, here's my buffer construction picture:

[caliber dia.]

I made a couple barrels in 5.45x39.

I used pac-nor blanks and I think they were 0.221"

Mags were the real issue though.

I never got the ammo to stack properly.

This was a while back though so maybe the 6.8 mags will work with them.

C-Products was supposed to be making a specific mag for them, but I don't know how successful they have been.

I think you would need a mag with more curve to it, like an AK mag.

C-Products generally keeps the shape of the mag the same, but changes the internal rib thickness.

[Any decent 40-45 round mags that work?]

I dug out some PMI steel 40 round mags that I bought back in the 90's recently to play with.

These mags are heavier steel than the typical gunshow crapola.

They came packed in some nasty grease and in plastic bags.

Once degreased, I put magpul followers in them.

I have run them in a dozen matches or more without one single failure of any sort.

They are no longer available, but if you find genuine PMI mags, they are good mags.

[HK AR15 Magazines and Lightened Bolt]

I had 3 bolt over bullet malfs during a recent match while using HK mags in a cinch.

The gun has an MSTN lightened bolt.

The worn USGI cinched mags that HK mags were supposed to replace works 100%.

Was this just a fluke or do the HK mags not have enough spring pressure to work with a lightened bolt?

Ultralight carriers often need a slighty larger gas port to get full function.

If you bought the whole upper from MSTN, it should have already been setup properly.

If you add a lightweight carrier and buffer to an existing gun, the carrier might not function correctly because it does not carry enough momentum.

You want the buffer to just kiss the inside of the buffer tube.

There's about 1/4" more bolt travel than needed to pickup the next round.

If the bolt does not travel that last 1/4", there will not be enough TIME for the next cartridge to present itself to the bolt.

If you get misfeeds where the gun just needs a jolt, you are under-gassed slightly.

Increasing the bolt travel a bit will give the bolt carrier some momentum before it tries to strip a round off the mag.

[ar gas block]

Welcome to the forums Randall!

Been here two years, just never posted.

I'll probably be making more of a presence here though.

[ar gas block]

is the gas block on a barrel with a short hand guard and sight in the same place as the gas lock on a mid length hand guard

Mid-length gas blocks are exactly 2" further forward on the barrel.

Read through my gas system explanation for more on why this is so important.