Author Topic: Swipes  (Read 274 times)

Offline Constructor

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Swipes
« on: July 17, 2018, 10:47:49 AM »
What causes swipes?
Swipes can be caused by a few things but in order for swipes to occur the case MUST be stuck to the chamber walls and the carrier must travel to the rear far enough to turn the bolt 22.5 degrees where it unlocks. It is not always high pressure as far as being over max spec chamber pressure.
 It could be because the pressure is high and over-gassed because of it. The carrier moves before the case can shrink back down.
 It could be that the case is sized a little long and it being pinched between the bolt face and shoulder keeping the case from turning. I once read where a guy(Yondering I believe) that frequents another forum sizes his cases for a .008" crush fit in an AR chamber. That is just asking for trouble.
It could be that the neck of the case is thick and the case can't turn in the chamber. It could be that the gas port in the barrel is a little larger than it needs to be for that load.
It could be because the melonite chamber is sticky. New Melonite chambers get sticky when first heated up. They will smooth out after broken in or the chamber can be lightly polished.
Some look at posts online and wonder why they can't get the velocity others are getting without getting swipes. It isn't always high pressure like most believe. It is timing more than anything with the exception of sizing the cases incorrectly. An adjustable gas block can fix the issue some times. Others above are self explanatory. Primers make a difference. Powder burn rates make a difference. Using a faster burning powder will cause a pressure spike early. Using magnum primers will light more of the powder at once and cause a pressure spike. In 07 when I was testing the 6.8 so much I figured out I could get more velocity by using Russian primers and more powder. A smaller spark ignites less powder slowing the pressure spike in some cases.
« Last Edit: July 17, 2018, 11:34:02 AM by Constructor »
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Offline kindabitey

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Re: Swipes
« Reply #1 on: July 17, 2018, 11:43:51 AM »
 Outstanding post. One thing I've found that has helped, is the LMT E-bcg and bolt. It delays the unlocking of the bolt slightly to allow the pressure to drop (We're talking milliseconds). The bolt also fully supports the cartridge base by having the extractor machined around the bolt face area as opposed to being cut out for the extractor. Not sure how they make this happen, could be witchcraft or some type of sorcery  :o. All I can tell you is, without one, I would have never gotten 150 gr. Lehigh match solids over 2000 fps out of my 18" 300 blk.

The downside of them is:
1- They're not cheap, at all
2- You won't see pressure signs nearly as soon as a standard BCG. They behave more like a bolt action.
3- Did I mention they're expensive?

https://lmtdefense.com/parts/l7q3c
https://lmtdefense.com/parts/l7r3a

That's almost $390.00 for a bolt carrier and bolt, but if I start feeling the need to really push things, I'll get one.
Maliois- French for "You don't want one"
Because it's fun, and I can, that's why

Offline Constructor

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Re: Swipes
« Reply #2 on: July 17, 2018, 12:03:26 PM »
The cam pin track is longer so the carrier travels to the rear more before it starts turning the bolt.
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Offline JDB1980

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Re: Swipes
« Reply #3 on: July 17, 2018, 02:46:23 PM »
If this is the same bolt as the LWRC 5.56/.224 Valkyrie bolt, Brownell's "Prime Time Blow-Out Sale" has the bolts for $54.59 today....normally around $145. I am using two (2) of these and have no complaints....other than the extractor pin being a little difficult to remove/reinstall due to the pressure exerted by the two springs in the "whale tail".

Offline Xaddo

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Re: Swipes
« Reply #4 on: July 17, 2018, 10:36:51 PM »
I just realized how little I know about ARs! I've been shooting a Grendel for 8+- years but it's the only AR I've owned.. still is sort of is as it was the donor for my Valkyrie.I can take it apart and put it back together 100% but the posts above were out of my wheelhouse . Thank you guys for the info I'm gonna try to process it!

Offline kindabitey

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Re: Swipes
« Reply #5 on: July 18, 2018, 07:03:01 AM »
 I'll scratch around to see if I can find the article on the bcg. I'm pretty certain it's not the same as the LWRC, since they're different companies.

This is from M4 carbine. I just copied the text, didn't think all of the other was important. This is from Bill Alexander:

Let us perhaps be a little more precise with regard to the details of the LMT enhanced bolt and carrier system. As shown the details are very often misinterpreted.

In the first instance one must understand why the system exists, more particulaly in the context that the design is brilliant in its execution, but as reported frequently on the errornet is not always reliable. The design basis for the assembly is specifically to enhance the durability of the M4 configuration as currently issued. This gun is to some extent unbalanced. Port pressures running military grade ammunition are well in excess of the design levels which leads to very high carrier acceleration rates (+22 f/s/s). this combined with chamber pressure drop often creates the undesirable condition that the bolt commences to unlock while still subject to a degree of head thrust from the cartridge case. The result is that the lugs are subject to a biaxial loading of both shear and bending.

The LMT system being both the carrier and the bolt seeks to operate directly in this gun and elleviate symptoms. The carrier is set up with a longer delay during the initial portion of its movement. To facilitate this longer cam path and movement of the bolt forwards in the carrier, the front edge of the carrier is extended such that it still continues to retain the extractor pivot pin. Additional exhaust vents act to drop the piston pressure faster and to relieve any blowby at the tail of the bolt.

The bolt itself is of particular interest. The function of the dual spring extractor is frequently misinterpreted as an attempt to add spring force to the extractor claw. Rather it reduces the fatigue that the extractor spring(s) undergo by allowing the use of longer springs with lower K values; the % relative compression during the movement of the extractor is reduced. Remember that additional extractor force is not required now that the carrier is slowing the extraction cycle. The mitigation of stress in the bolt is accomplished in several ways. Material is the least visible change but is important to the design. The traditional Carpenter 158 is abandoned, being replaced by a significantly tougher grade from a different manufacturer. The lugs themselves are generously radiused between lugs and at the rear the diameter is actually reduced to allow a larger transition radius to be machined. The incorrectly identified sand cuts on the lugs are stress relief cuts. These allow any individual lug to elastically deform and give a smoother load over the contact patch. While this type of feature is very difficult to calculate and even more difficult to implement it helps to place the lug in a true shear load rather than amplify the bending moment. As noted the lug opposite the extractor is relieved. This feature prevents the unequal transfer of load to the two opposite lugs but I would argue that the stress relief groove already in place largely accomplishes this purpose. This is a academic quibble so I will bow to LMT in this respect. There is one additional feature that can be found in the bolt, but I am not at liberty to disclose the detail.

When considering wether to use the LMT parts one must consider the weapon. Correctly ported guns will derive little or no benefit from a carrier that is specifically set up to absorb excessive port pressures and some degree of residual case pressure. If not sufficiently gassed the reliability will suffer. This is not a fault of the carrier rather a mistake made in the application so be careful with simple substitutions. The bolt itself is exemplary. If not constrained by a $ value the bolt is a worthy addition to any rifle and will do nothing but enhance the durability of this part of the system.

Bill Alexander

Here's a link to the thread, if you want to read the entirety:

https://www.m4carbine.net/showthread.php?3379-LMT-Enhanced-Bolt-Question
« Last Edit: July 18, 2018, 07:58:51 AM by kindabitey »
Maliois- French for "You don't want one"
Because it's fun, and I can, that's why

Offline kindabitey

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Re: Swipes
« Reply #6 on: July 18, 2018, 08:05:48 AM »
I just realized how little I know about ARs! I've been shooting a Grendel for 8+- years but it's the only AR I've owned.. still is sort of is as it was the donor for my Valkyrie.I can take it apart and put it back together 100% but the posts above were out of my wheelhouse . Thank you guys for the info I'm gonna try to process it!

I'll agree with that 100%. It seems the more I learn about the AR platform, I'm finding out how little I know. I am very fortunate that I'm able to call on experienced professionals for information and trouble shooting, and highly trained folks that use these on a daily profession for issues to keep an eye out for. The majority of these folks, you've never heard of, and they prefer it that way.
Maliois- French for "You don't want one"
Because it's fun, and I can, that's why

Offline towerofpower93

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Re: Swipes
« Reply #7 on: July 19, 2018, 04:21:50 AM »
I have one of the (as far as I know) no longer produced LMT Super Bolts in my 7" 7.62x39MM upper.

When I sent it to Ron Williams to try and get an 11.5" piston gun running, we went back and forth about my wants and the realities of physics. Ended up with a 7" gas gun and the LMT bolt was the only one able to survive the violence of the action.

Never knew why it was designed so differently, but Mr. Alexander's explanation makes sense to this layman.

Thanks for posting that kindabitey.
« Last Edit: July 19, 2018, 03:46:43 PM by towerofpower93 »