Are some Green Mountain .25 Marauder barrels choked and some not?

[subscriber]

I have some H&N Grizzly "slugs" that measure .2495" diameter before shooting.  They measure .248" across the groove diameter near the rear of the projectile after shooting them into modelling clay from my stock Gen2 Synthetic Marauder.  The hollow nose mushrooms a bit, so I avoid measuring near that area.

This suggests the barrel may have one or two "mills" of diametral choke.  On the other hand, I can't see any choke; so it may simply be that diameter all the way through.  The bore looks mirror bright and the rifling impressions on pellets captured in clay appear crisp.

I don't want to remove the barrel from the gun unnecessarily, but think that would be required to "slug" the barrel properly.

Any info about typical dimensions for Green Mountain barrels would be appreciated.   If you have measured yours, what land and groove diameters did you find?  Choked or not?

Chamber dimensions would also be appreciated.  My impression is that first 3/8" from the breech is "generous" in diameter.  Perhaps as large as .258" based on how freely .250" pellet heads move when inserted by hand. 

The abrupt clicks (one for the head and one for the skirt) when seating pellets slowly with the probe suggests that the leade is cut rather steeply.  Perhaps somewhere between 30 and 45 degrees.

Thanks

[FuzzyGrub]

It was my understanding that all the 25 cal mrod GM barrels were choked.  I have no measurements, but my Gen 1 (sold) and Gen 2 (purchased new), have a noticeable choke when pushing a pellet through.  I have a used GM barrel that also has a choke, but not as pronounced.  Given it was used, didn't know if the prior owner tried to make more slug friendly or was like that from the factory.   I'll be curious to what others say.

[Dairyboy]

All the ones I've owned were choked

[triggertreat]

I have two GM barrels and both are choked.

[maraudinglizard]

Mine is choked. There is a member on here that could give you some answers to your questions. Give him a pm. Also I have searched for him and his posts, these should help as well.

https://www.gatewaytoairguns.org/GTA/index.php?action=profile;u=6340

https://www.gatewaytoairguns.org/GTA/index.php?action=search2

[K.O.]

I think there is some variability to them... how much I do not know I only have two... I also have 2 XL 725 barrels  and one is slightly tighter than the other. they are not choked and rough enough to need lapping and the twist rough measured is about (tight patch method)1:19...

So the .25 Mrod barrels well I only have looked at the one (6/16 Synrod)... Cause the other (4/15 wood stocked Synrod) does so well it makes me want to leave it alone.

But I have a feeling the 4/15 barrel like almost all Crosman Barrels it could benefit from some leade work... Especially for slugs...

pellets are pretty forgiving and even a 45* leade would be an improvement on the leade of the 6/16 Barrel...yep straight up and sharp edged like all crosman barrels that I have had (pumpers and PCP)...(the XL 725 barrels not even slightly beveled and of course no chamber being a spring rifle)

The chamber is very slightly larger than groove size on mine... you can see it in the pic at reply #15 in the below thread

https://www.gatewaytoairguns.org/GTA/index.php?topic=121437.msg1184497#msg1184497     

what I get  is right about .250/.252 Bore/Groove on both barrels... choke seems to be about .249 on the 4/15 barrel... about .248-.2485 on the 6/16 barrel which also at about 15" in tightens very slightly and stays tight to the choke...

Did some leade(90-95% done about 2-3* angle) work on the 6/16 barrel to be able to use the BBT 40g and it helped... need to do it just a touch better for the BBT... but it really helped it with the Benji 27g domed... did not like them at all before now sends them better than the untouched 4/15 barrel...

My measurements are with a Fowler micrometer that claims +/- .00016 accuracy... how good I am with it is up for debate..

http://www.fowlerprecision.com/Products/Vernier-Micrometers/0-1-Outside-Inch-Micrometer-52-229-201.html

I just recently got a .249 sizing insert so will be testing some .249 out... right now the BBT 40g do best sized to .2495 without knurling...

so just from my 2 barrels I think there may be a bit of variance in the amount of choke between barrels... but yep the choke is much milder than the L.W.'s  about .243 choke...

[subscriber]

Thanks folks.

I ordered a decent .22 cleaning rod.  Will chamber a few slug and press them back out with the rod to see how they measure.

[subscriber]

I chambered a number of pellets by means of the probe, and then pushed them out with a cleaning rod from the muzzle.  My Mitutoyo calipers measured the apparent groove diameter of these pellets at .2535 to .254".  Mostly .254".

I don't have any confidence in my attempt at land diameter measurement because the impressions seem narrower than the caliper's "beak" tips.  It is probably reasonable to assume bore diameter would be about .006 less than groove diameter; as in .248".

Then I loaded a pellet backwards to get a feel for the leade angle.  Other than the high bolt closing force indicated the angle was "steep", I had no information.  So, I decided to remove the barrel to get closer look at the leade.  That would also allow me to push pellets all the way through the barrel to get a choke measurement.

Removing the barrel was much easier than I expected.  It took less than 15 minutes, and I was being careful.  I am actually a little concerned about the small size of the four screws that hold the receiver to the air tube.  First time you use a Marauder as a club, those screws are going to let go

Pellets pushed past the choke measured .252". Now, these measurements seem large to me.  My confidence in them is based on that they were taken with my newest calipers, with just a clean zero confirmation.  I am pretty sure they are correct to 0.0005", but suspect pellets may "spring back" a little, and that actual barrel diameters may be slightly smaller.

Anyway, starting pellets with a cleaning rod seemed to take more force than with the probe.  Once the pellets have "popped" loose they seem to travel with quite a bit of friction for a few inches.  After 6" or so, the resistance has dropped to an estimated 1 to 2 lb.  Then just over 2" from the muzzle, the pellets "come to stop that requires about as much force to move again, as starting them when initially chambered. 

What is weird is that pellets seem to move freely again about 3/4" from the muzzle.  Then they stop again as if there is a second choke; before popping out. 

I did not push pellet in a just few inches beyond the chamber and then reverse them for measurements, although by the feel of things, that might be useful.

Now, before I removed the airstripper, I thought the crown looked OK (picture below).  However, removing the airstripper revealed a bright ding right on the edge of the bore line (two pictures).  The general constricted feel of pellets at the muzzle suggest there may more of a burr all the way around that should be removed.

I tried to take some pictures of the chamber and leade (shown below).  What is obvious is that the leade angle is not a sharp 90 degrees.  It seems to be somewhere between 30 and 45 degrees.

Unfortunately, the transfer port cuts into the lead "ramp" at the bottom of the chamber, leaving a sharp edge.  Judging by lead adhering to the leade in places, there is some roughness here that explains the high degrees of force required to start a H&N  Grizzly "slug"; especially backwards. 

This would seem to predict that the leade should be at least be deburred and polished before any kind of solid projectiles will load and shoot satisfactorily. 

I do not have a one degree reamer laying about, so I will try more primitive methods to polish the lead, without harming the rifling.

I will also try the brass screw method to clean up the crown first.  I don't think I this will risk making it worse that it is now.

I suspect that lots of shooting in dusty environments would polish the leade and "fire-lap"  the barrel into more uniformity, including deburring the muzzle.   Rather than wait that long, or use course "fire-lapping" media right off the bat, I figured I would tackle the crown and leade more directly.

I don't know if pellet friction dropping off 6" from the chamber indicates the bore opening up, or if that is just the pellet wearing down in a parallel bore...

Any thoughts about fire or hand lapping this barrel to improve it should be coupled with the idea of having a spare on hand; in case the improvements don't work as planned...

[Motorhead]

I use CRATEX points sized to the bore / leade being worked with. With barrel in lathe And cratex point in tailstock  can feather in the rifling very precisely.  Worst case done by hand with Cratex point in a dremel tool and lightly inserted and bumped up against rifling can give similar results.

Another cratex point shaped to a angled face cleans up the crown at muzzle nicely too.

[subscriber]

Thanks Motorhead,

Where do you get these Cratex bits?

Meanwhile, I ordered these 1/4" polishing bits from McMaster:
https://www.mcmaster.com/#4958A45
https://www.mcmaster.com/#4958A43
https://www.mcmaster.com/#7803T26


As you say, the idea would be to just bump up against the rifling, rather than force a long section of compliant material to enter the bore...

I figured that even the harder plastic bob would be less likely to "go too far".  That even the cylindrical shaped ones would conform to the end of the chamber enough to polish, without altering the leade angle much.  The cone ended one would have to be used very gently...

[KnifeMaker]

Thanks Motorhead,

Where do you get these Cratex bits?

Meanwhile, I ordered these 1/4" polishing bits from McMaster:
https://www.mcmaster.com/#4958A45
https://www.mcmaster.com/#4958A43
https://www.mcmaster.com/#7803T26


As you say, the idea would be to just bump up against the rifling, rather than force a long section of compliant material to enter the bore...

I figured that even the harder plastic bob would be less likely to "go too far".  That even the cylindrical shaped ones would conform to the end of the chamber enough to polish, without altering the leade angle much.  The cone ended one would have to be used very gently...

I usually order Cratex bits from Brownells Gunsmithing supply. They are available in different grits  and are easily shaped to task. 

You will be there forever trying to shape the mcmastercar bobs down to size. LOL

Knife

[subscriber]

Thanks KnifeMaker

I plan on running those 1/4" diameter bobs into the chamber from the large hole at the end of the barrel.  For polishing the inside of the port, there are smaller bobs available.

[Motorhead]

Points or Bobs ... Lol do also come in various hardness of rubber compounds the abrasive is suspended within.
Shaping best done while spinning and held against a grinding wheel dressing brick or most accurately done against a running 1" best sander.

Go slow and learn to use these ... There extremely effective for the task at hand, but do remove material !!!! so go slow and keep contact pressure low until you get a feel for how much bite they have. 

[subscriber]

Good advice!

Go slow and learn to use these ... There extremely effective for the task at hand, but do remove material !!!! so go slow and keep contact pressure low until you get a feel for how much bite they have.

[K.O.]

Hi sub

I tend to think that your measurement are way off and that that the leade appearing to be angled is a product of how you took the picture...

I am not trying to be a "Richard Cranium" it is just that most vernier calipers are not very accurate and not for when very accurate measurements are needed... most are accurate to +/- .002 or .001 at best  depending on the model (including Mitutoyo calipers)... Heck I think that pin gauges are better than a micrometer... and for accuracy's sake do not let your hands warm them...

 about leade angle

the leade begins at the muzzle side of the barrel port hence you can not take the pic from directly above (90*) hence it will make it appear to be angled depending on the angle of your viewpoint...

about the Bore/groove diameter that is the specs for the .25 Lothar Walther barrels  .248/.254

http://www.lothar-walther.com/458.php

L.W. barrels have at least twice the land height of the  .25 Mrod barrels... The fact that the Green mountain barrels do not have a lot of land height is not a big deal these are pellet barrels the are designed for low fpe...about 50...

look at SAMMI specks for .22lr  and you will see .217/.222 for bore/groove... in reality many have less land height .219/.222... and that is for 100+ fpe... Heck Marlin Microgroove is about .0015 with 16 narrow lands...

some BBT 40g push thru pics reply #8 in this thread   

https://www.gatewaytoairguns.org/GTA/index.php?topic=120618.0

[subscriber]

Hi KO.

I took some pictures of the leade through the port, closer to perpendicular with the barrel.  They were not clear, so I did not bother posting them.  The leade angle may not be 30 to 45 degrees, but is very clearly is not a 90 degree step.  Anyway, pellets seem to hesitate before their skirts even get to the leade, so a bit of general smoothing out should help.

The funny thing with my calipers is that I get the same readings between multiple calipers.  Due to the pellet skirt being so thin, I probably pressed a little softer than if it were a cylindrical slug.  However, taking up all the slack without springing or yielding anything is what you are supposed to do.

When I measure gauge pins with my calipers I get their nominal value.   A close the calipers, then look at the reading.  I don't watch the reading and push until I see what I want.

Now, I don't have any gauge pins  in size increments smaller than 0.0005", which is the nominal limit for my Mititoyo.  So, I agree that a decent micrometer would be much better, but I think my caliper measurements are good enough to classify the barrel.  Based on what you said, it looks like I have a LW barrel 

[Bill G]

Hi KO.

I took some pictures of the leade through the port, closer to perpendicular with the barrel.  They were not clear, so I did not bother posting them.  The leade angle may not be 30 to 45 degrees, but is very clearly is not a 90 degree step.  Anyway, pellets seem to hesitate before their skirts even get to the leade, so a bit of general smoothing out should help.

The funny thing with my calipers is that I get the same readings between multiple calipers.  Due to the pellet skirt being so thin, I probably pressed a little softer than if it were a cylindrical slug.  However, taking up all the slack without springing or yielding anything is what you are supposed to do.

When I measure gauge pins with my calipers I get their nominal value.   A close the calipers, then look at the reading.  I don't watch the reading and push until I see what I want.

Now, I don't have any gauge pins  in size increments smaller than 0.0005", which is the nominal limit for my Mititoyo.  So, I agree that a decent micrometer would be much better, but I think my caliper measurements are good enough to classify the barrel.  Based on what you said, it looks like I have a LW barrel 

Are your using over or under pins.  You calipers won't detect the diviation, you need a .0001 micrometer.   If you measuring device is expected to measure a given resolution of measurement, it needs to read 50% of the expected resolution. Your calipers displays .0005 thus are good for .001 accuracy.  With the pins, they should be .0002 +/- depending on the declaration (over or under).  Is the barrel supposed to be factory barrel? I may have missed this in a previous post, if so I apologize. A full set of ZZ class pins with cabinet will run you about a grand. That's for the cheap inports .011" to 1".   

[Bill G]

I see you said," my marauder barrle".  It is generally thought that factory Marauder barrels are Green Mountain Barrels.

[Bill G]

LOL!!! I suppose I should have read the entire post. Sorry.

[subscriber]

Thanks Bill;

Some of my gauge pins are over and some under.  Got them from McMaster.  Calipers agree with the gauge pins' nominal diameters.  These were bought in 0.0005" increments. Never see caliper readings off from gauge pins, so combined errors seem less than another full 0.005".  Or I am just lucky.

Even if I am only measuring to the nearest 0.001" my (assumed Green Mountain) barrel seems to have a .254" groove diameter, rather than the expected .252".  Even if I am 0.001" off (and the gauge pins suggest I am not even .0005" off, my barrel seems like an anomaly.

That said, I agree that to have confidence in a measurement that one is going to sell, your measurement system needs to be 10 times better than your tolerance (or 5 times in a pinch).  Mine is clearly not better than +-0.0005", but I have measured a few parts in my time; so it is not as bad as +-0.001".

Calipers that have too much slop in the "beak" can cause larger errors.  This usually shows itself as a lack of repeatability.  If I can't get the same reading multiple times then either I am holding the part skew, or the caliper is worn or mall adjusted. 

I admit that measuring skirted pellets is tricky, but holding them skew tends to make the measurement smaller rather larger.  My measurements are suspect because they are larger than expected, rather than smaller.  Yet they are not suspect, because they repeat