Thank you to our advertisers!







Author Topic: How much jump before pellet hits the grooves?  (Read 2096 times))

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #80 on: November 15, 2021, 08:30:36 PM »
Greg,

One way to locate a specific diameter on an odd flute taper reamer is to drop "washers" onto it.  If you measure the washer ID first and see where it settles on the reamer that should provide useful info.

If extrapolating from the reamer angle to find the location for the .358" diameter is too difficult and inaccurate, then ream out the washer with the reamer first.  Measure the tightest diameter in the washer to verify it is .358", then drop the washer onto the reamer to mark the start of the relief taper.  This reaming and measuring may require a few tries; and perhaps more than one washer...

All you need are some brass or copper washers, although mild steel is hardly going to hurt the reamer.  You can leave the pre-sized washer on the reamer while you relief grind the reamer; then use the washer as a comparator (after wiping off the bulk of the dust). 

You can even have two (or more washers) at .359 and .357 as GO and NOGO gauges.  The GO needs to be able to pass the max OD at the end of the relief grind; while the NOGO should not.  By also having a .360" sized washer on the reamer, you can use that to know when you are close and need to start paying attention with your relief grinding.

Just another idea...
« Last Edit: November 15, 2021, 08:33:08 PM by subscriber »
  • USA

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #81 on: November 15, 2021, 08:45:18 PM »
One recurring thought I have about taking an accurate Cerrosafe bore impression:  Before you take a chamber cast; check out the new barrel's factory chambering job with your bore scope first.  Convince yourself that there is no burr where the factory cone meets the bore.  If there is a burr there, that will make removing the chamber cast harder; and more significantly, scrape or swage down the cast to a smaller than actual diameter.  A 0.0005" error may not matter; but I think a burr or displaced steel 0.002" tall might.

If you see a burr or ridge where the new barrel's cone meets the bore, a very light polishing may be indicated to remove it.  Then your land and groove diameter info will be more accurate, to determine better pilot and max chamber OD dimensions.
  • USA

Offline Greg_E

  • Sharp Shooter
  • ****
  • Posts: 921
  • yes
  • Real Name: Greg
Re: How much jump before pellet hits the grooves?
« Reply #82 on: November 15, 2021, 11:03:23 PM »
I already had to clean up that new barrel... It even had some light rust in there. I have a little car wax on it now, should help prevent rust from returning.
  • USA, NY, Syracuse

Offline Spin

  • Plinker
  • **
  • Posts: 179
  • yes
  • Real Name: Jerryy
Re: How much jump before pellet hits the grooves?
« Reply #83 on: November 16, 2021, 12:09:10 AM »
At some point in the not too distant future I'm going to sell off some of my tools and I can tell you there are some things you guys would likely be interested in.
  • USA, WI. Chilton

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #84 on: November 16, 2021, 12:25:46 AM »
Lathe and mill, Jerry?
  • USA

Offline Greg_E

  • Sharp Shooter
  • ****
  • Posts: 921
  • yes
  • Real Name: Greg
Re: How much jump before pellet hits the grooves?
« Reply #85 on: November 19, 2021, 10:14:28 PM »
Not pretty, but should be functional. I still need to knock the corners off the bigger end so they can't cause any damage, that will get the size where I need it. I was originally going to grind the entire large end on my mill contraption, but it was obvious that the amount of work was not worth it for a part that will never go into the barrel. Some tape to protect things from the cutting edges until I can grind those on a bigger grinder. I'll do things a little differently if I decide to change the spiral flute reamer, it was a lot of work to get here and I can reduce that amount of work with some alternate thinking.
  • USA, NY, Syracuse

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #86 on: November 19, 2021, 11:11:16 PM »
Looks like it should work, Greg

I am not sure what else needs to be ground?

The relief at the large end only needs to be a little deeper than the chamber you want to cut. 

As with most things, it is more work and more time consuming than the simple nature of the project would suggest.

I was tempted to PM you yesterday for an update, then decided that doing so might be annoying, and just to wait and see.   :)
« Last Edit: November 19, 2021, 11:13:30 PM by subscriber »
  • USA

Offline Greg_E

  • Sharp Shooter
  • ****
  • Posts: 921
  • yes
  • Real Name: Greg
Re: How much jump before pellet hits the grooves?
« Reply #87 on: November 20, 2021, 12:47:42 AM »
My mill lives at work right now, so time is limited. Really need to make a space for it at home.

I'll grind a lot less on the spiral reamer next time, then bench grind the excess since it doesn't need to be as good that far back.
  • USA, NY, Syracuse

Offline darkcharisma

  • Expert
  • *****
  • Posts: 1487
  • yes
    • DCtunes
  • Real Name: duy
Re: How much jump before pellet hits the grooves?
« Reply #88 on: November 20, 2021, 01:13:32 AM »
So this is the reason why my .172 Impact went from 1030fps down to 950fps. all because i cut the breech shorter and cut the leade where the rear band enages the lands upon chambering. 28.15 down to 27.5 barrel length too.

 i thought my Impact broke! The Jump leade gave 2 inch wide prints at 100 yards.
  • SACHSE

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #89 on: November 20, 2021, 01:15:33 AM »
Duy,

That sounds like something else changed.  Not sure what.
  • USA

Offline darkcharisma

  • Expert
  • *****
  • Posts: 1487
  • yes
    • DCtunes
  • Real Name: duy
Re: How much jump before pellet hits the grooves?
« Reply #90 on: November 20, 2021, 01:47:34 AM »
besides the 3/4 barrel chop and the no jump leade, i am sure there is nothing else unless the gun's reg just went haywire.

Going with Bobs formula I should get 63FPE, which is 1030fps with 26.6 grains. Now it's not. Only 940FPS at best
  • SACHSE

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #91 on: November 20, 2021, 01:54:39 AM »
Is there any way for the thimble to rotate (or be installed back to front), to partially obscure the TP?

Springers use "no jump" leades and work better that way.  I presume you are shooting slugs.  Perhaps the leade angle is too steep and too rough, thus generating a lot more friction.  Now, a running start does operate at lower friction (dynamic friction VS static friction), but the difference in efficiency seems over the top.

Not that you want to keep cutting the barrel shorter, but what about engaging only the front drive band on loading, and not to full depth on the lands - so still on the "ramp"?  On the other hand, if the driving bands are fully engraved on loading, most of that work is done during loading.  For a split second the static friction would be high, but less work would be done than engraving the lands via air pressure.

The rear of the loaded projectile should not obstruct the TP either...
« Last Edit: November 20, 2021, 02:01:35 AM by subscriber »
  • USA

Offline darkcharisma

  • Expert
  • *****
  • Posts: 1487
  • yes
    • DCtunes
  • Real Name: duy
Re: How much jump before pellet hits the grooves?
« Reply #92 on: November 20, 2021, 02:59:48 AM »
The rear of the slug does stick out about 1mm obstructing the TP . and It was going 1030fps when it as just at the edge of the TP.

So I am guessing 3 changes since 1030fps.

I decided to cut down the barrel to start a fresh leade to get the best accuracy. Might not have been the best choice
  • SACHSE

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #93 on: November 20, 2021, 05:27:31 AM »
One inch off 28" won't even drop you to 27/28 of the original FPE; unless the valve was held open till the projectile left the muzzle.  You have lost about 18% FPE.  The barrel length reduction is only about 4%, so something else is going on.
  • USA

Offline rkr

  • GTA Senior Contributor
  • ******
  • Posts: 4077
Re: How much jump before pellet hits the grooves?
« Reply #94 on: November 20, 2021, 11:54:16 AM »
So this is the reason why my .172 Impact went from 1030fps down to 950fps. all because i cut the breech shorter and cut the leade where the rear band enages the lands upon chambering. 28.15 down to 27.5 barrel length too.

 i thought my Impact broke! The Jump leade gave 2 inch wide prints at 100 yards.

Did the accuracy improve with increased contact to rifling?
  • Finland
Huub Viking Mk2 .177/.22 bullpup - grab'n go gun
BSA Scorpion SE .177 - 12 fpe UK model
BSA Scorpion .25 - 100M BR gun at 60 fpe
BSA Scorpion .224 - 100M BR gun at 100 fpe
Evanix Blizzard .257/.357 - 160/230 fpe
Exanix Sniper X2 .357/.45 - 210/270 fpe silhouette gun
Drozd Blackbird HPA - 1200 rpm full auto fun gun / meat grinder
Evanix AR6 carbine/pistol
+ a couple of springers

Offline Greg_E

  • Sharp Shooter
  • ****
  • Posts: 921
  • yes
  • Real Name: Greg
Re: How much jump before pellet hits the grooves?
« Reply #95 on: November 21, 2021, 12:19:06 AM »
The straight flute reamer is a fail. It keeps falling into the grooves and trying to take a big bite. I'll have to get the spiral flute cut down and try again.
  • USA, NY, Syracuse

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #96 on: November 21, 2021, 12:33:31 AM »
The straight flute reamer is a fail. It keeps falling into the grooves and trying to take a big bite. I'll have to get the spiral flute cut down and try again.


It is a left hand spiral, right hand cut, right?  With 7 flutes, should not grab onto the lands like the straight cut reamer.

A right hand spiral reamer may be worse that a straight flute one.  Especially if the flute count matches the number of lands or a multiple thereof...
  • USA

Offline Greg_E

  • Sharp Shooter
  • ****
  • Posts: 921
  • yes
  • Real Name: Greg
Re: How much jump before pellet hits the grooves?
« Reply #97 on: November 21, 2021, 01:54:20 AM »
I counted wrong, it is 6 flutes too. Left spiral, right cut.
  • USA, NY, Syracuse

Offline subscriber

  • GTA Senior Contributor
  • ******
  • Posts: 4701
  • yes
Re: How much jump before pellet hits the grooves?
« Reply #98 on: November 21, 2021, 02:01:12 AM »
The left hand spiral creates a force trying to "spit the reamer out".  That helps reduce its tendency to grab.  You just have to apply very little force pushing the reamer in to cut; especially at the start.

I think having the barrel vertical with the reamer at a comfortable height is the way to approach this.  The force you apply is almost just enough to keep it stable, more than showing it in to cut.

If the barrel is held in a vice, have the jaws at least 2 inches away from the reamer pilot, because a thin walled barrel will spring out of round, and you want to cut it far away from the temporary oval section.  Else, you will cut an oval into it - seen after removal from the vice.  This ovality can be reduced by using close fitting padded vice jaws.  And / or staying away from between the jaws.

It might help to use grease (or Vaseline) as the cutting lube, until more contact area is established.

Is the rifling a right hand spiral?  That should help with a left hand spiral reamer.
« Last Edit: November 21, 2021, 02:40:26 AM by subscriber »
  • USA

Offline darkcharisma

  • Expert
  • *****
  • Posts: 1487
  • yes
    • DCtunes
  • Real Name: duy
Re: How much jump before pellet hits the grooves?
« Reply #99 on: November 21, 2021, 02:55:44 PM »
So this is the reason why my .172 Impact went from 1030fps down to 950fps. all because i cut the breech shorter and cut the leade where the rear band enages the lands upon chambering. 28.15 down to 27.5 barrel length too.

 i thought my Impact broke! The Jump leade gave 2 inch wide prints at 100 yards.

Did the accuracy improve with increased contact to rifling?

I am trying to sort the tune out first. once i get it back to 1000fps i will shoot 100 yards and let you know.

Thanks Sub for reaching out via PM.
  • SACHSE