This is to clarify any misleading information about the LMT Enhanced Bolt and it's purpose. This information comes straight from LMT. Thank You,  I have no affiliation with LMT.
Steve,

The enhanced carrier incorporates a longer twell time in the cam path.  The additional time will allow the internal barrel pressure to be greatly reduced.  When the bolt starts to rotate, the brass case isn't held against the chamber walls under press -- allowing easier extraction.  This will increase more when the rifle is equipped with a flash suppressor.

External features have also been improved to reduce friction and aid in functionality in harsh conditions.  Specialized plating helps with reduced friction and increased resistance to corrosion.

The enhanced components were designed for the 14.5" M4's that historically showed problems associated with ejection issues and poor performance at high temperatures.  The components were not designed for a 16" or 10.5" barrel.  That's not to say they won't function in a different barrel length, but there may not be improvement in function.

Quoted: This is to clarify any misleading information about the LMT Enhanced Bolt and it's purpose. This information comes straight from LMT. Thank You,  I have no affiliation with LMT. Steve, The enhanced carrier incorporates a longer twell time in the cam path.  The additional time will allow the internal barrel pressure to be greatly reduced.  When the bolt starts to rotate, the brass case isn't held against the chamber walls under press -- allowing easier extraction.  This will increase more when the rifle is equipped with a flash suppressor. External features have also been improved to reduce friction and aid in functionality in harsh conditions.  Specialized plating helps with reduced friction and increased resistance to corrosion. The enhanced components were designed for the 14.5" M4's that historically showed problems associated with ejection issues and poor performance at high temperatures.  The components were not designed for a 16" or 10.5" barrel.  That's not to say they won't function in a different barrel length, but there may not be improvement in function.

All makes sense, except "twell".  Is twell supposed to be dwell?

Quoted: The enhanced carrier incorporates a longer twell time in the cam path.  The additional time will allow the internal barrel pressure to be greatly reduced.  When the bolt starts to rotate, the brass case isn't held against the chamber walls under press -- allowing easier extraction.  This will increase more when the rifle is equipped with a flash suppressor. External features have also been improved to reduce friction and aid in functionality in harsh conditions.  Specialized plating helps with reduced friction and increased resistance to corrosion. The enhanced components were designed for the 14.5" M4's that historically showed problems associated with ejection issues and poor performance at high temperatures.  The components were not designed for a 16" or 10.5" barrel.  That's not to say they won't function in a different barrel length, but there may not be improvement in function.

That makes no sense whatsoever. I hope they mean sound suppressor?!.........

Also don't forget, according to LMT the enhanced bolt was also designed for M855 so if it doesn't work for other ammo then tough luck...

i have an enhanced bolt in a 10.5 and shoot 55 grain ammo and have almost 5000 rounds without a problem so far.

LOL, the info regarding the carrier is correct.  The bolt however can be used in a standard carrier.  Any talk about it being designed for the M855 is pure B/S.  This bolt is used in the LW 499 and the pressure curve on that cartridge is dramatically different to any 5.56mm round - it functions correctly.  
HFG

Quoted: Any talk about it being designed for the M855 is pure B/S.  HFG

No kidding...

Quoted: Quoted: Any talk about it being designed for the M855 is pure B/S.  HFG No kidding...

Yes it is.  Especially when it comes from LMT as an excuse for why an LMT factory assembled upper receiver with an LMT bolt and bolt carrier is unreliable with XM193 ammo.

Are you talking about the enhanced BOLT or BOLT CARRIER?

The enhanced bolt I have won't run for shit in my 10.5". Runs great in my 14.5" w/M193.

Go figure.

Doc

Quoted: Are you talking about the enhanced BOLT or BOLT CARRIER?

I'm talking about the dual spring LMT enhanced bolt.  Feeding Cannibal had issues with his LMT enhanced bolt and he was told by LMT that it was made specifially for 62gr M855.  WTF!

To this day I have not heard how one goes about to tuning a bolt for a specific bullet weight.  AFAIK, if you're using the correct type of ammo and it's loaded properly so that it can cycle the action the bullet weight has no effect on reliable function.

To me a bolt is a bolt is a bolt.  If it's manufactured properly with good gas rings and extractor spring(s) then it should work properly with any ammo  that has proven reliable with your rifle/carbine.

all this info should have been available prior to the release of  the enhanced b.c.g.
not after it 's been sold. it's completed b.s.
all i remember was  it was advertised as the ultimate reliabilty b.c.g.
like i said b.s.

Quoted: Quoted: Are you talking about the enhanced BOLT or BOLT CARRIER? I'm talking about the dual spring LMT enhanced bolt.  Feeding Cannibal had issues with his LMT enhanced bolt and he was told by LMT that it was made specifially for 62gr M855.  WTF! To this day I have not heard how one goes about to tuning a bolt for a specific bullet weight.  AFAIK, if you're using the correct type of ammo and it's loaded properly so that it can cycle the action the bullet weight has no effect on reliable function. To me a bolt is a bolt is a bolt.  If it's manufactured properly with good gas rings and extractor spring(s) then it should work properly with any ammo  that has proven reliable with your rifle/carbine.

You may already know all of this, if you do, forgive me... but here is a short overview of some of the issues and efforts to shore things up:

First, the M4 has its problems... no question and they are well documented. Several things come into play, but one of the big issues is the peak pressure inside of the bolt during operation (about 2.5 kPSI) and the resulting excessive bolt velocity.

On result of the higher bolt velocity is the extracter *popping* off of the round during extraction, to take a look at what is really happening here, we have to keep in mind that the extracter pin is not located in the center  as far as mass goes, it is unbalanced, like this:

=======o=================

With the "o" being the pin location in the extracter.

As you can see, there is more mass forward of the pin than aft, as the bolt moves reward, it rotates... not much (20 degrees iirc), but it does spin pretty fast - - so fast in fact in the M4 that centrifical force causes the forward (hook) part of the extracter to try to escape orbit by moving away from the center, reducing the amount of hold it has on the cartridge.

The dual spring idea compensates, sort of... but a better idea is to move the pin closer to the center of balance -- Don't run out and jump on Solid Works, it has already been developed (more on that if you are interested).

Now, for the "long dwell" carriers... again an idea that has been around for a while but not really that popular -- the camming of the bolt is a *triangle* of functions, angle of rotation, lock time and travel of the bolt carrier... the angle of rotation can not be changed, so in order to increase lock time, the travel of the bolt is increased.

As stated, the effort is to keep the bolt locked up longer, one problem with the M4 is that the bolt is unlocking early... while chamber pressures are still high -- this is not really an effort to tune the bolt for a specific bullet weight, rather  for a specific pressure curve... a pressure/time curve that is far more than the rifle was designed to work with.

So, the modified cam timing of the bolt is designed to mitigate the problems of this specific gas port pressure (ammo particular) and the rather long time function of the M4's P/T curve.

A couple of issues with altering the established dwell... as seen here with lower pressures and shortened time functions, and as I recall from earlier efforts the rifles had some reliability issues in cold weather.

Anyway... sort of a primer to the issues and reasons to things discussed here, hih?

Gunzilla,

Yes, I'm aware of the "Extractor Lift" issues with the M4 and I also understand why extending the dwell time is good for reliablity especially with the short carbine gas system.  Though none of these issues and fixes have any relation to the weight of the bullet being fired.

Either way you wrote up an excellent post because I'm sure there are probably a lot of people reading this thread wondering what the hell we're talking about...

Quoted: Gunzilla, Yes, I'm aware of the "Extractor Lift" issues with the M4 and I also understand why extending the dwell time is good for reliablity especially with the short carbine gas system.  Though none of these issues and fixes have any relation to the weight of the bullet being fired. Either way you wrote up an excellent post because I'm sure there are probably a lot of people reading this thread wondering what the hell we're talking about...

First off thanks... I used to have a rather lengthy diatribe about the issues with the M4 and why things go south on that platform on my web site... maybe I will put it back up?

Anywho, I think that the reference to the *bullet weight* is a misnomer given to the specific P/T curve of M855 -- anyway, I still say it is treating the symptom rather than the problem.

Quoted: Gunzilla, Yes, I'm aware of the "Extractor Lift" issues with the M4 and I also understand why extending the dwell time is good for reliablity especially with the short carbine gas system.  Though none of these issues and fixes have any relation to the weight of the bullet being fired. Either way you wrote up an excellent post because I'm sure there are probably a lot of people reading this thread wondering what the hell we're talking about...

Actually, bullet weight CAN affect both the pressure curve AND the dwell, both of which factor in to the cycling of the weapon. Ask the Garand shooters what happens to their rifles if they try to push heavier 180gr bullets too hard, attempting to find a useful long range load. A heavier slower bullet CAN have BOTH a higher pressure AND a longer dwell time, adding more stress to the system.

I'm no engineer, in ANY way shape or form, but the problems are fairly obvious to me, even if I don't have the technical ability to clearly describe them.



Lonny

Gunzilla,

I can see what you're saying about the P/T curve of the M855 but my feeling is that a non-defective LMT enhanced bolt in a standard carrier should be able to work reliably with ammo that has been proven to work in the same upper with a standard bolt.  I personally think LMT was blowing smoke up Feeding Cannibal's ass and should have just replaced his enhanced bolt with another one that worked.  JMHO...

Quoted: Actually, bullet weight CAN affect both the pressure curve AND the dwell, both of which factor in to the cycling of the weapon. Ask the Garand shooters what happens to their rifles if they try to push heavier 180gr bullets too hard, attempting to find a useful long range load. A heavier slower bullet CAN have BOTH a higher pressure AND a longer dwell time, adding more stress to the system. I'm no engineer, in ANY way shape or form, but the problems are fairly obvious to me, even if I don't have the technical ability to clearly describe them. Lonny

Good points, now please explain to me how the LMT enhanced bolt is supposedly designed to "work reliably with M855"?  I'm not talking custom loading ammo to extremes here, I'm wondering about 55gr XM193 and 62gr M855 and the BS excuses that was given about the enhanced bolt...

If you or someone else really knows  the design specifics please educate this poor old boy.  I'm not being smart, I really do want to know how it's done.

Yojimbo is right, I had issues with the LMT enhanced bolt using a standard LMT carrier.  LMT told me that bullet weight was the problem.  I returned it, because I want a bolt group that will work with a variety of bullet wieghts reliably.  Now using a Colt with McFarland rings and a Wolff extractor spring.  Zero issues, and a helluva lot cheaper.

Quoted: Yojimbo is right, I had issues with the LMT enhanced bolt using a standard LMT carrier.  LMT told me that bullet weight was the problem.  I returned it, because I want a bolt group that will work with a variety of bullet wieghts reliably.  Now using a Colt with McFarland rings and a Wolff extractor spring.  Zero issues, and a helluva lot cheaper.

+1

The thing that I find interesting about designing a bolt around functioning with a specific round is this: the presumption that M885 is going to be the ammo de jour until the end of all time

Does it work with Mk 262 Mod 1 for instance? What if the military as a whole decides to go with something other than M885?

Quoted: Gunzilla, I can see what you're saying about the P/T curve of the M855 but my feeling is that a non-defective LMT enhanced bolt in a standard carrier should be able to work reliably with ammo that has been proven to work in the same upper with a standard bolt.  I personally think LMT was blowing smoke up Feeding Cannibal's ass and should have just replaced his enhanced bolt with another one that worked.  JMHO... Quoted: Actually, bullet weight CAN affect both the pressure curve AND the dwell, both of which factor in to the cycling of the weapon. Ask the Garand shooters what happens to their rifles if they try to push heavier 180gr bullets too hard, attempting to find a useful long range load. A heavier slower bullet CAN have BOTH a higher pressure AND a longer dwell time, adding more stress to the system. I'm no engineer, in ANY way shape or form, but the problems are fairly obvious to me, even if I don't have the technical ability to clearly describe them. Lonny Good points, now please explain to me how the LMT enhanced bolt is supposedly designed to "work reliably with M855"?  I'm not talking custom loading ammo to extremes here, I'm wondering about 55gr XM193 and 62gr M855 and the BS excuses that was given about the enhanced bolt... If you or someone else really knows  the design specifics please educate this poor old boy.  I'm not being smart, I really do want to know how it's done.

The LW.499 pushes a 300 grain bullet with 32,000 psi and uses the LMT bolt - it works fine - now that's about as different as it gets from anything 5.56.  
HFG