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| | |-+  .308 MDR barrel extension and cycling (important)
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Author Topic: .308 MDR barrel extension and cycling (important)  (Read 4442 times)
Blackandwhiteknight
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« on: May 17, 2018, 01:04:36 PM »

  I know title sounds clickbait-ish, but if you are having cycling issues with your MDR you need to hear this.  I've got about 900 rounds through me MDR so far and have had recurring issues with the bolt failing to seat completely, leaving it out of battery.  At first it seemed sporadic so I figured it was ammo, or bad headspace.  Last outing while doing accuracy testing with 5 rounds in a mag I was getting a failure to go into battery almost every mag.

  Well I was going over the rifle with a fine tooth comb looking for headspace issues and found the culprit. The barrel extension, more specifically the feed ramp.

  Cycling the bolt by hand very carefully with dummy rounds I found they dragged really bad going up the left feed ramp.  Right here.



Picked up the dummy and found this.



I thought, "you gotta be kidding me!".  That's right, a massive bur was ripping the snot out of all the rounds going up the left feedramp.  Roughly 50% of the rounds I have been firing have been tweaked, and badly marred.

  Closer inspection of the feedramp.  Top of the left ramp, inside.  You can see the shreds of brass ripped from the jacket.



It looks like sloppy machining.  The grooves for the bolt lugs look to be cut after the the feedramp leaving a nasty hook like bur coming back off the edge.  Well, this wasn't caught and cleaned up before it was Nitrided, and basically got locked in stone.  Close to 900 rounds and this bur hadn't budged a bit.  Nitriding both the bolt and extension is great if the parts are in spec, and smooth. It's a recipe for disaster if they aren't.

 The polishing fairies magically appeared and cleaned up the lugs, and the feedramp for me. Grin  Feeds smooth as glass now.  Think it helped the locking lugs as well.

  So if you are having any issues with the cycling you need to go over the barrel extension, locking lugs and feed ramp with a fine tooth comb.  These burs can easily effect the feeding ability, and the locking/ unlocking of the bolt.

ColdboreMiracle  if you still monitor these forums you need to pass this on.  You guys are shooting yourself in the foot here.  These tool marks need to be cleaned up before these parts go to Nitride!
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thehun
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« Reply #1 on: May 17, 2018, 01:13:03 PM »

Good catch...beta testing is almost complete...keep going...

How does it feel to do a quick and dirty DIY gunsmith work on your $2500 MDR...by the way...you voided the warranty on your MDR now by performing an unauthorized repair.
« Last Edit: May 17, 2018, 01:15:01 PM by thehun » Logged
Ditcher
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« Reply #2 on: May 17, 2018, 02:05:24 PM »

by the way...you voided the warranty on your MDR now by performing an unauthorized repair.

They will never know lol
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rtp
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« Reply #3 on: May 17, 2018, 02:29:22 PM »

by the way...you voided the warranty on your MDR now by performing an unauthorized repair.

They will never know lol

Untrue - they might say the barrel looks too good, so must be modified or aftermarket.  Grin Angry Angry Angry
Good catch OP, will take a look at mine.  Have certainly seen a few scratches similar, but tough to differentiate when also chasing stuck cases etc.
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kfeltenberger
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« Reply #4 on: May 18, 2018, 12:51:34 AM »

FWIW, I checked the barrel on the MDR that I haven't assembled.  There is a faint copper wash/coating on the edges of each of the two sets of guides.  I took a a Q-Tip and ran it along the path that the bullet would take, thinking that if there was a jagged edge or burr, that it would snag a few strands of the wrapped cotton; it didn't.  So, for me, that copper on the extension could simply be wear as both a Q-tip and tactile examination didn't disclose a burr.

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Kurt
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« Reply #5 on: May 18, 2018, 08:02:34 AM »

Could possible those burrs be caused by the bullet tip slamming into the extension?
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kfeltenberger
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« Reply #6 on: May 18, 2018, 09:01:47 AM »

Could possible those burrs be caused by the bullet tip slamming into the extension?

The extension would have to be pretty soft...softer than aluminum.
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Kurt
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« Reply #7 on: May 18, 2018, 09:42:54 AM »

True.

Does FAXON make the extensions as well or just the barrels...anyone know?

Might not be FAXON's QC issue if the extensions are sourced out to someone else.

However no matter what...final QA is done at DT...and that stuff should be caught prior to leaving the factory...final quality that ships out to customers stops with DT.
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thehun
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« Reply #8 on: May 18, 2018, 12:19:44 PM »

In reality...DT should be the ones telling us what is causing these issues...they alone have the most data points available and engineers at hand for proper diagnosis.

But they have been crickets about all this...so...we all are just spinning our wheels trying to provide feedback to the community...like good BETA testers.
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Blackandwhiteknight
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« Reply #9 on: May 18, 2018, 01:56:28 PM »

True.

Does FAXON make the extensions as well or just the barrels...anyone know?

Might not be FAXON's QC issue if the extensions are sourced out to someone else.

However no matter what...final QA is done at DT...and that stuff should be caught prior to leaving the factory...final quality that ships out to customers stops with DT.

With the manufacturing capability and experience that Faxon has there is no way the barrel extension came from them.  On mine you could literally see the bur start at the edge of the machined cut for the lug as it was pulled out of the hole.  For a company that has done Aerospace work this level of "oops" would be almost unthinkable.  And I agree, there is no way DT should be missing things like this.  A guys cycling dummy rounds and checking finishes would find this stuff easily.
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rtp
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« Reply #10 on: May 18, 2018, 03:12:01 PM »

I'm pretty sure the barrel extensions come from DT.
I believe I've heard that references both for SRS 3rd party barrels/conversions as well as somewhere in the MDR 6.5 conversion discussion.. ??
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Ditcher
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« Reply #11 on: May 18, 2018, 03:19:50 PM »

I'm pretty sure the barrel extensions come from DT.
I believe I've heard that references both for SRS 3rd party barrels/conversions as well as somewhere in the MDR 6.5 conversion discussion.. ??

I think you're right cause if I remember correctly the guy that was working on the 6.5C got the extention from DT.
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thehun
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« Reply #12 on: May 18, 2018, 04:02:46 PM »

Oh man.  ROFLMAO ROFLMAO ROFLMAO ROFLMAO ROFLMAO If they are indeed from DT...

Seriously...what happened? DT turns out...from the most part...great bolt guns...this quality shift to the MDR is crazy.

It feels like they don't care about final quality and they just cranking rifles out to make that greedy dollar.
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Sdevante
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« Reply #13 on: May 18, 2018, 05:05:00 PM »



It feels like they don't care about final quality and they just cranking rifles out to make that greedy dollar.

They are just trying to avoid having to say "2 MOAR WEEKS" any further, I think.....even though now that is going to be their warranty department's slogan.
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TNC
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« Reply #14 on: May 18, 2018, 06:10:57 PM »

OP, you may or may not be on to something there.  The brass/copper residue on the feed ramps in itself is not unusual.  They are ramps, and it's not at all uncommon to see little flecks of brass/copper residue on them and inside the extension after operation...unless you're using pure lead or other type of bullet besides "clad" bullets.  Likewise it's not that uncommon to see small scratches on the bullet itself after hand cycling.  There is an acceptable and an unacceptable level of marking, of course, and I don't know what DT deems acceptable.

On the feed ramps in your pics, it's hard to tell from the visual they represent.  You're mentioning noticeable burrs and irregularities you can see and feel in person, and that's something that obviously can't add to smooth cycling.  The design of the ramps doesn't really look that unusual, but burrs and non-symmetric irregularities would surely not be intended.
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mityno1
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« Reply #15 on: May 19, 2018, 07:10:30 PM »

Feed ramp friction and gouging issues might explain why some ammo ejects better than others:

1) The hardness or softness of the projectile and even the casing would likely cause one brand/type of ammo to incur more or less damage during chambering than another brand/type of ammo

2) OAL, seating depth and projectile ogive and overall shape would also likely contribute to one brand/type of ammo suffering more or less damage during chambering than another

3) Setback / Push Back / Neck Tension... ammunition is loaded to a specific set of specifications in an attempt to deliver consistent performance parameters. While it is obvious that the powder charge plays a dominant role in that equation, so too does projectile weight, cartridge neck tension, and projectile setback.  If the projectile is being pushed back into the cartridge because it is not making a smooth transition into the chamber, then the pressure will rise or even spike.

SIMPLE MDR TESTS:

Start with a box (or several) of the same ammo. Measure the OAL of this ammo.

Gently chamber a round by PLACING IT IN THE CHAMBER WITH YOUR FINGERS ever how you can go about accomplishing it.

Fire that round and observe whether it ejected correctly.

Repeat several times.

Now, gently chamber it round by placing it in the chamber with your fingers. Next, load a second round in a mag. Load that mag into the MDR so that two rounds are available to be fired, the one in the chamber and the one in the mag.

Fire only one shot, the chambered round.

Eject the second unfired round and inspect it unfired. Measure and compare the OAL. Was it pushed back when it chambered?

Repeat a few times.

Lastly, use a sharpie marker to mark the brass on a round and gently chamber it round by placing it in the chamber with your fingers. Next, use a sharpie marker to mark another round on its brass with a different mark and load this second marked round in a mag. Load that mag into the MDR so that two rounds are available to be fired, the one in the chamber and the one in the mag.

Fire both rounds. If you have a crono, use it.

Inspect the brass you marked with a sharpie.

These two rounds should look identical and have very little velocity difference if you are able to measure it.

If there is no difference in placing a round in the chamber with your fingers (no matter what you have to do to achieve it) and a round chambered by the semi-auto action, then the feed ramps are probably not the problem.

But if the feed ramps are the problem, you are not going to fix them with a different gas valve.

Here is my problem with all of this... 200 rounds on "Adverse" to wear in the components?

Think about it.

WTF is that all about?

Exactly what parts are we supposed to be wearing in and why couldn't this have been taken care of in the manufacturing process? 

Obviously, DT knows something is out of spec or there would be no need to "Adversely" over gas the rifle for 200 rounds to wear out whatever it is that is out of tolerance.

Which brings me to my question...

Does DT know the feed ramps are rough, irregular and inconsistant and their solotion is 200 rounds on Adverse to "break in" the feed ramps? 

I dunno, that's why I'm asking.
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Re: MDR Group Buy - official BPF list!
Reply #360 on: January 04, 2015, 05:29:50 AM
(Still waiting patiently on an FDE...)
kfeltenberger
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« Reply #16 on: May 19, 2018, 07:46:47 PM »

Why not just coat several rounds in dykem and then cycle them through the rifle?  That will quickly show any drag points and spotlight where they are on the cartridge. 
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Kurt
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« Reply #17 on: May 19, 2018, 08:45:12 PM »

Why not just coat several rounds in dykem and then cycle them through the rifle?  That will quickly show any drag points and spotlight where they are on the cartridge. 

If you shoot the rifle to let it chamber your dykemed round from the mag and then eject it unfired. OAL still needs to be checked and compared. 

If the projectiles are not being pushed back into the case, then there is less likelihood the feed ramps are the reason for damaged rims.

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Re: MDR Group Buy - official BPF list!
Reply #360 on: January 04, 2015, 05:29:50 AM
(Still waiting patiently on an FDE...)
mityno1
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« Reply #18 on: May 20, 2018, 02:31:36 AM »

I want to emphasize there are three different way a cartridge can be chambered in a semi-auto and each may have a different effect on both the ejection cycle and on accuracy:

1) Full semi-auto operation - A round is stripped from the magazine and chambered as a result of the bolt's return cycle from that is powered by the firing of the previous round.

2) Manually charged operation - A round is stripped from the magazine and chambered by manually pulling the bolt back and:

2a) releasing the charging handle to let it "fly" home into battery.

2b) "riding" the charging handle home to control or otherwise slow its forward travel into battery.

3) Inserting a round into the chamber with one's fingers and then closing the bolt into battery.

Each of these three modes of chambering a round will more often than not produce a different POI.

Many rifles that tend to be FRF guns (FRF = First Round Flier) have this tendency often attributed to a cold bore condition when it may well be the first shot has a different POI because the first round was manually stripped from the mag with the charging handle. Bolt guns don't have this issue because they don't have a violent incredibly fast semi-automatic cycle. The point is, how a round is chambered can make a big difference because it can affect how a semi-auto cycles.

I would think that at a minimum, the first topmost round in the magazine should be marked to identify it so that when the brass is inspected, it can be noted if any rim damage is different from those that were chambered manually with the charging handle and those chambered far more viloently with much greater force by the semi-auto action.

If there are differences in the first manually charged round and those cycled semi-automatically then either:

1) The first round will show more damage to the rim/case.

or

2) The following semi-auto cycled rounds will show more damage to the rim/case.

If there are fewer or no problems when chambering rounds with your fingers the feed ramps may be a contributing factor.

If more of the problems are with the first manually charged round, and placing the first round in the chamber with your fingers fixes the first round issues, there is only a small chance there is an issue with the feed ramps.

If more of the problems are with the semi-auto cycled rounds than the first manually charged round, then the odds are high there are feed ramp issues contributing to the extraction issues.

But if the issues remain consistent, no matter which of the three methods are used to chamber the round, then the feed ramps are probably not a significant factor contributing to the problems. 

I went into mag testing techniques in another thread, but I will give a short version here:

1) Do all of the above with a topped off mag.

2) Repeat all of the above with a mag downloaded at least two rounds but more than 1/2 full.

3) Repeat all of the above with a mag that only has no more than 3-4 rounds loaded in it.

This will test to what degree the magazine spring tension is having on getting the rounds chambered.

Ultimately I would at least try to identify which combination of all of the above conditions made the extraction problems appear worse or not as bad. Or at least show me that none of the above tests had any effect on my extraction problems.

That is because extraction problems can be the result of chambering issues and vice-versa.

One cycle of firing one round begins with chambering and ends with extraction.

And one cycle of the bolt going back and forth extracts one round and chambers another.

In other words, the bolt is powered by the round being extracted and this power is what chambers the next round.

So a failure at any point in this cycle can manifest as a symptom of another point in the cycle.  The key is to divide the problem by testing each chambering method to see if there is a cause and effect on the extraction and/or rim.casing.

No cause and effect being noted limits how much can be attributed to the feed ramps/barrel extension.

A significant amount of cause an effect difference between the method of chambering and the extraction points to, but does not totally confirm that the feed ramps/barrel extension may be a factor.


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Re: MDR Group Buy - official BPF list!
Reply #360 on: January 04, 2015, 05:29:50 AM
(Still waiting patiently on an FDE...)
thehun
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« Reply #19 on: May 20, 2018, 09:37:33 AM »

Why on earth are owners the freaking engineers for DT...lol...this is so sad.
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