Author Topic: No more GM barrels in Mrods? (Read 13948 times))

T3PRanch · « **Reply #120 on:** July 14, 2019, 04:15:13 PM »

Currently listed as OUT OF STOCK!

oldpro · « **Reply #121 on:** July 14, 2019, 04:16:59 PM »

I have a few laying around both new and used. PM me if your looking for one.

wheeling_hunter · « **Reply #122 on:** July 16, 2019, 12:02:21 PM »

What kind of groups is everyone getting with the mrod 25 cal. Crosman barrels at 50 yards?

triggertreat · « **Reply #123 on:** July 16, 2019, 03:02:14 PM »

Quote from: wheeling_hunter on July 16, 2019, 12:02:21 PM

What kind of groups is everyone getting with the mrod 25 cal. Crosman barrels at 50 yards?

I haven't recorded any groups in awhile, but here are all of the groups with the GM barrel I have recorded, back when I first got my .25 cal, or when tuning shortly afterwards.

The first groups/pic are at 25 yards, but the rest are at 50 yards. I may have been more focused on turning than groups, but this should give an idea. Some groups are when I was just testing different pellets, too. I settled with the MKIIs and is all I shoot these days. They have proven very worthy since and provide the most thump, which I like.

wheeling_hunter · « **Reply #124 on:** July 21, 2019, 11:07:13 AM »

That GM barrel is a shooter I hope the Crosman barrels does the same

USAFANG6799 · « **Reply #125 on:** July 21, 2019, 11:18:10 AM »

Quote from: wheeling_hunter on July 21, 2019, 11:07:13 AM

That GM barrel is a shooter I hope the Crosman barrels does the same

One would think that Crosman would be standing on the highest mountain yelling this at the top of their lungs if it were.

FuzzyGrub · « **Reply #126 on:** July 22, 2019, 01:34:33 PM »

Raining here this AM, so had a few minutes to do the first measure. This is the crosman 25 barrel. It has 6 lands and 6 grooves.

For twist rate I get 1:19". I did it with a half rotation or 0.5:9.5"

Using the gauges I got from Steve, get 0.246" for the choke and it is a tight 0.246". They do not go deep enough to get a measure of the bore, but the choke is what I wanted to know about.

Since I had the barrel off, I opened the TP to 0.187" and deburred.

PS: I did notice some circular tooling marks about a 1/2" past the TP. They look like they had been smoothed out.

Next up will be a GM barrel from my Gen 2 Mrod, which was one of the first 100 sold.

Machinist · « **Reply #127 on:** July 22, 2019, 01:49:00 PM »

Rats....

I made the gages .650" long.

How long do they need to be? The longer they are, the more difficult it is to make.

I can send you longer gages, and a .2455" to get a little closer on the choke.
Steve

FuzzyGrub · « **Reply #128 on:** July 22, 2019, 02:21:19 PM »

GM #1: (from my Mrod)

Choke: 0.247" the pin is not quite as tight as crosman
Rifling: 10 lands and grooves
Twist Rate: 1:17"

GM #2: (purchased used, years ago from Mrod forum member)

Choke: 0.247", the pin is not quite as tight as crosman, but same as GM #1
Rifling: 10 lands and grooves
Twist Rate: 1:18"

These were both measured in gun, from muzzle, and only explaination I have why I got different twist rate.

The crown is different on the crosman 25 barrel (left most) as you can see in picture below.

FuzzyGrub · « **Reply #129 on:** July 22, 2019, 02:26:12 PM »

Quote from: Machinist on July 22, 2019, 01:49:00 PM

Rats....

I made the gages .650" long.

How long do they need to be? The longer they are, the more difficult it is to make.

I can send you longer gages, and a .2455" to get a little closer on the choke.
Steve

Steve,

Some numbers to compare to what you got, anyway.

I'm not sure on the right length, but my guess would be 1" or slightly longer. That should get back to the tightest part of the choke, if 0.650" wasn't far enough, and get into the bore, a half inch past the TP.

Machinist · « **Reply #130 on:** July 22, 2019, 04:16:15 PM »

John,

A 245" diameter gage goes into the muzzle freely. A .246" diameter gage is tight in the muzzle. The .246 would not fall out of the bore by gravity alone right?

So the actual size of the bore at the muzzle is somewhere between .245 and .246"

If I send you a .2455" diameter gage, you can narrow down the size of the choke a little more.

Will the .2455" gage go in freely?

Steve

FuzzyGrub · « **Reply #131 on:** July 22, 2019, 04:34:15 PM »

Quote from: Machinist on July 22, 2019, 04:16:15 PM

John,

A 245" diameter gage goes into the muzzle freely. A .246" diameter gage is tight in the muzzle. The .246 would not fall out of the bore by gravity alone right?

So the actual size of the bore at the muzzle is somewhere between .245 and .246"

If I send you a .2455" diameter gage, you can narrow down the size of the choke a little more.

Will the .2455" gage go in freely?

Steve

In the crosman barrel, the 0.245 is loose, and the 0.246 is very tight. You have to push it in with a bit of force.

Using my HF calipers, the one I labeled 0.246 measures 0.2455, and the 0.246 and 0.247 are reading 0.0005" low. That is probably the calipers.

I think we can say the crosman is 0.001" tighter in the choke than the GM.

Machinist · « **Reply #132 on:** July 22, 2019, 07:10:11 PM »

John

We can definitely get the bore size to within .0002/.0003"

I measured your gages with a Mitutoyo micrometer that has a .0001 Vernier scale. I went a step better with Keith's gages and measured those with an indicator mike. You can see 20 millionths with that. I clearly marked the size on his aluminum gages. I marked the size on yours too, but the writing is pretty light.

The plastic being kinda soft isn't the best gage material. It really gives you just a rough idea. Wanna try a brass gage? Or PEEK, I have some PEEK. Not sure how hard the PEEK is compared to Delrin. The Delrin can be deformed pretty easily.

I was thinking about how I measured the twist rate. I aligned the wrench/indicator with a cheap square I got at the hardware store. It could be off by a degree or maybe more. My eyes could be off a little too. And my lathe has nicks that could cock the square a little. Add it all up and it makes a difference. I figured if I really had 177 degrees of rotation, instead of 180 degrees, the twist rate would change by over half an inch. Since I only measure a half revolution and then multiply by two, any error I have is also multiplied by two. A degree wheel would give a better reading.

Steve

FuzzyGrub · « **Reply #133 on:** July 22, 2019, 07:31:50 PM »

Steve,

Alum or brass would be fine. Whichever is easier for you. I will just have to be careful and not push it in, like I did with the Delrin.

I re-chronied the crosman 25 barrel with JSB 25g, and seems like it loss about 20fps from what it was with GM. Not that much, and could be something else minor that changed over time.

It will take allot of bore paste to take the choke down 0.001"

triggertreat · « **Reply #134 on:** July 23, 2019, 07:16:17 PM »

I just received the rods from Steve.

On my spare GM barrel (10 lands). The .245 is the only one that will slide into the muzzle end. it is a snug fit. On the .250 leade end, .248 is the last one that will enter the lands past the leade. So, .248 barrel lands with .245 choke lands. 1:19 twist.

On my installed GM barrel, I'm getting the same results at the muzzle end as I expected, but is not quite as tight to insert the .245. This maybe due to having thousand of rounds through it compared to the unused spare. These two barrels are roughly of the same date range. About three to four years old now.

Machinist · « **Reply #135 on:** July 23, 2019, 07:49:38 PM »

Keith

How about we try a .2485" gage in the breech end? Or a .2486 and a .2488? Get a little closer.

Glad the gages were long enough to reach past the chamber. I machined off my chamber to cast the lead slugs. Didn't measure it before I turned it into chips. I guessed at how long to make the gages.

I thought the Green Mountain barrels were 1:15.5 twist?

Thanks for the info.
Steve

triggertreat · « **Reply #136 on:** July 23, 2019, 08:00:09 PM »

Quote from: Machinist on July 23, 2019, 07:49:38 PM

Keith

How about we try a .2485" gage in the breech end? Or a .2486 and a .2488? Get a little closer.

Glad the gages were long enough to reach past the chamber. I machined off my chamber to cast the lead slugs. Didn't measure it before I turned it into chips.

I thought the Green Mountain barrels were 1:15.5 twist?

Thanks for the info.
Steve

I'm game. It's whatever you feel like doing. I do thank you for your time and efforts!

As far as twist, read this thread. I have checked mine myself, but there are several pros that have done so as well and offered their input on this thread:

https://www.gatewaytoairguns.org/GTA/index.php?topic=152675.25

Machinist · « **Reply #137 on:** July 23, 2019, 08:54:18 PM »

I just read the twist rate thread. Thanks for the link. I think.

If we passed around one barrel, I wonder what kind of range the twist rate would have?

Maybe Chubby could help us?

porkchopexpr3ss · « **Reply #138 on:** July 31, 2019, 08:08:21 PM »

.25 purchased from crosman 4/9/18 has the GM 10 land barrel. finally checked.

Machinist · « **Reply #139 on:** August 01, 2019, 07:14:42 PM »

Hey Clark

Thanks for the info. I bought my 25 Mrod barrel from crosman in August of 2018, and it has 6 grooves. So the change occurred between April and August of 2018

Steve

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No more GM barrels in Mrods?

Author Topic: No more GM barrels in Mrods? (Read 13948 times))