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Author Topic: Am I thinking right?  (Read 270 times)

Nvreloader

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Am I thinking right?
« on: October 08, 2017, 12:07:43 PM »
Guys,
I am short on experience here, so I am requesting some help on chambering a new bbl of my QB 79,22 cal.
I am starting from scratch on this bbl. I am pencil whipping a diagram to machine too,   :-\

Please refer to this diagram on post #16, this diagram is close to what I want to do, THANKS Bob..
http://noebulletmolds.com/smf/index.php?action=profile;u=1665;area=showposts;sa=topics;start=15

Here is the info I am working with, taken from the repeater breech/pin bolt/bbl measurements,etc
in reference to the above diagram.

The starting port hole diameter, will be .140" dia, (OEM) and may be opened up to .170" diameter later,
(depending on what the results of testing revels etc), and it has to be .280" (hole center) from the end of the bbl,
this diameter is allowing for the port hole to be enlarged to .170" (this length dimension can't be changed) etc,

The bolt pin pushes the pellet base (inside of the skirt base) into the bbl about .550",

I want to use the 2nd diagram and start the lead angle, just ahead of the port hole edge,
and I hope the longest skirt depth should end up just ahead of the edge of the lip of the lead, and port hole edge.

Or should I use the first drawing diagram, which would be the easiest, as long as the skirt edge is past the port hole edge,

Here are the measurements of the DEPTH of pellet skirts,
Inside depth of skirt, (from the bottom edge of the skirt), with .092 pin diameter, of the repeater bolt.

Shortest
Benjamin Super Point =============----.052"
JSB Match Exact Jumbo Monster Diabolo==----.058"
JSB Match Ultra Shock Heavy Diabolo====---.064"
JSB Match Exact Jumbo Heavy Diabolo ===---.065"
Skenco Ultrashock Hollow Point========---.072"
Benjamin Hollow Point =============---.076"
Crosman Premier HP ==============---.084"
Benjamin Pointed Expanding =========---.094"
Benjamin Domed Magnum ==========----.096"
Crosman Premier Piranha ===========---.098
JSB Match Diabolo Straton Jumbo ======---.100"
Kodiak Tapered Dome=============----.108"
H&N Baracuda Match =============----.108"=5.51 Head Dia
JSB Match Exact Jumbo Diabolo =======---.112"
H&N Baracuda Match==============---.112"=5.53 Head Dia
Gamo Red Fire =================----.114"
Skenco Newboy Hollow Point =========---.136"
H&N Field target Trophy ============---.142"=5.54 Head Dia
Skenco Newboy Junior=============----.144"
H&N Field Target Trophy============----.154"=5.55 Head Dia
Ruger Superpoint================----.170"
Gamo Platinum==================---.200"
Longest

Shortest Base depth = .052"
Longest Base depth = .200"
Difference of .148" in skirt depth.

Any help would be greatly appreciated.

Thank you,
Don
« Last Edit: October 08, 2017, 07:44:48 PM by Nvreloader »


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It is time to restore the American precept, that each individual is accountable for their actions"
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"Speeds fine, but Accuracy is final"

rsterne

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Re: Am I thinking right?
« Reply #1 on: October 08, 2017, 04:07:59 PM »
With a 0.170" port centered 0.365" from the end of the barrel, the front edge of the port is 0.450" from the breech end.... If the probe is 0.550" from that same point, any pellet with a base depth greater than 0.100" will be partially obscuring the port when loaded.... If you can, you need a longer bolt probe....

As we discussed on the phone, the angle and position of the leade is not very important with pellets.... It is more important that the full depth rifling be undisturbed where the head of the pellet is located, for the shortest pellet, where that is closest to the breech.... You want the head fully engaged with the rifling when chambered with a pellet, because it tends to barely engage the rifling in most airgun barrels.... Leade angles for pellet barrels can be very steep, from 30-60 deg. per side, because the head engagement with the rifling is both short in length, and shallow....

Leades (ie the front of the parallel chamber) on pellet barrels can be anywhere from behind the port to just in front of it....

Bob
« Last Edit: October 08, 2017, 04:10:51 PM by rsterne »
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Nvreloader

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Re: Am I thinking right?
« Reply #2 on: October 08, 2017, 08:39:22 PM »
Bob

Thank for the info, (I tried to get the diagram to work, but it did NOT happen, on my end),

I made an error when looking at my rough draft, Sorry, (I hate these new glasses with the line right where you look).
I have corrected the error in my 1st post.   :-[

I just remeasured the port hole again, it is centered .280" from the end of the bbl.

This new repeater breech has a pin bolt, .092" dia, and there is NO way to change to a longer length pin,
because when the bolt is locked back in the 2nd bolt slot, (this bolt is a cock on open style, with no rear cocking pin),
which holds the bolt back for inserting the Mrod 10 shot mag, the end of the pin is recessed .010" below the back face/edge of the mag slot where it's inserted,
if the pin is made longer, I can't insert the mag etc. ( I don't see any way around it).

I will compare the OEM bolt (hollow nose) to the pin bolt, to see if there is any differences noted.

I'll set up, to cut a 45* angle for the lead, to start with, may change later, after pellet testing etc,
and I start with the pellet chamber of your first drawing, looks to be the simplest to start with.

Is there any other info I can provide, since you are doing the work and helping me figure this out?
which I greatly appreciate.
 
Thank you,
Don
  • Western NV
NRA Life member

"We must reject the idea that every time a law's broken, society is guilty rather than the lawbreaker.
It is time to restore the American precept, that each individual is accountable for their actions"
Ronald Reagan

"Speeds fine, but Accuracy is final"

rsterne

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Re: Am I thinking right?
« Reply #3 on: October 08, 2017, 10:12:40 PM »
With the new measurement.... you should be able to chamber any of those pellets (except Gamo Phantom) past the port, right?.... then you should be fine....

Bob
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Nvreloader

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Re: Am I thinking right?
« Reply #4 on: October 09, 2017, 09:56:30 AM »
Thanks Bob

By my calculations, (if correct), it shows that any pellet with a skirt depth of .200" or larger, will hang over the port edge,
and some thing else I just remembered, is the light tapering trough for the pellet passage around the port hole,
which will also change the distance/clearance also.

The port edge from the end of the bbl is .210", + .140" (OEM) dia, for the port hole = .350" to edge of port hole,
.550" is max pin depth, - .350" from .550" = .200" from the edge of the port hole,
but does not account for the tapering of the port hole, to assist the pellet passage etc. 

I am hoping, that I can find good accuracy with more of the most common pellets etc,
the inside operating length of the magazine, for the longest pellet length + accuracy,
will limit the pellet selection, and the single shot tray does not have any effect on the chamber length distance.

I haven't tried all the above pellets in the magazine yet, I will have to check the inside length for pellets, to see which fit/operate correctly etc.

Back to the drawing board,  ::)

Bob,
Why don't you start a online teaching course for the these AR/pistols,  ;)
as your experience is priceless in all these fields, PHD of AR's, = DR AR.

You are among the very few that seems to wants help anyone, IMHO.

Thank you,
Don

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"We must reject the idea that every time a law's broken, society is guilty rather than the lawbreaker.
It is time to restore the American precept, that each individual is accountable for their actions"
Ronald Reagan

"Speeds fine, but Accuracy is final"

rsterne

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Re: Am I thinking right?
« Reply #5 on: October 09, 2017, 10:52:43 AM »
Thanks for the kind words. Don.... happy to help.... I barely have enough time to get to my own projects, can't take on any more "Tutoring" than what I do on the Forums, tho....  ;D

Bob
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