Key hole- no key hole?

[willy]

Back ground =
Bought S&W model 28 barrel that had been bored and rifled to .430
Also found old beater model 28 .

Installed barrel and 44magnum cylinder on model 28.
Had to shorten back of barrel for cylinder gap .

Problem =
Shooting 250grain LSWC bullets over 10 grain of Unique I get about 2-3 perfect key hole and a couple bullets you can see was tilted when they hit the target out of 12 shots.

I shot groups with each cylinder to see if it was any one cylinder.
Same results.

So I switched to 240 grain JHP bullets
No problem

Next I tried 180 grain RNFP cast bullets.

No problems!!!

Why am I getting key holes only with the 250gr. SWC ?

Btw- The 250 swc shoots great in two other S&W revolvers I have.

[dondiego]

What is the diameter of the 250 grain bullets? What is the twist rate of the barrel?

[willy]

What is the diameter of the 250 grain bullets? What is the twist rate of the barrel?

Twist is 1-16”
Bullet dia is .431

[Bird]

Are you getting barrel leading?

[Larry Gibson]

Did you actually measure the twist or is that an assumption? I had a M28 converted to 44 SPL once and they use a slower rifle twist barrel [28" twist if I recall correctly] for the barrel liner. Would stabilize most all bullets up through 240 SWCs but not 429421s very well.

[Shopdog]

Am sure you did but gotta ask... did you range rod the new barrel and cylinder?

[Land Owner]

Problem =
...you can see...

No pictures? [...which may be out of the OP's hands as the site spins toward a different platform.]

A keyhole is "huge", by comparison. When at or near the center of the target it suggests an alternate means of "slugging" a bad guy with an unintended, but potentially lethal, meplat.

[willy]

Did you actually measure the twist or is that an assumption? I had a M28 converted to 44 SPL once and they use a slower rifle twist barrel [28" twist if I recall correctly] for the barrel liner. Would stabilize most all bullets up through 240 SWCs but not 429421s very well.

That is what I was told by seller ,,
I will check it out for myself

[willy]

Am sure you did but gotta ask... did you range rod the new barrel and cylinder?


No I did not,,but I figured if it shot other loads ,,then everything should be in the ball park

[fredj338]

Pin gage the cyl throats, you may be getting bullets swaged down. Did you slug the bore? May need a bit bigger bullet if its not 0.430”.

[willy]

That is what I was told by seller ,,
I will check it out for myself


As close as I can tell the twist is around 1-18..

[Larry Gibson]

Appears, at the velocity used, those 250 SWCs are on the cusp of stabilization. Same basic problem I had with my converted M28. I just picked up a Lyman 429360 (240 SWC) and they shot fine. Also the Lee 240 SWC and the TL 240 SWCs also shot fine.

[243winxb]

Was the forcing cone recut after barrel setback?

[willy]

Was the forcing cone recut after barrel setback?

Yes ,,

[willy]

Are you getting barrel leading?


I think you might onto something .
I just cleaned it and there was heave barrel leading in the forcing cone.

[243winxb]

Keyholes are caused by undersize bullets. The build up of lead in the forcing cone may swage the bullet smaller?

Try a stronger alloy. Add linotype. BHN 15 if you have a tester. Size to .430" same as groove diameter of barrel.

The 10 grs of Unique is 1 of my loads in the 44 mag, with 250 gr lswc.

Compare forcing cones to the other guns. See what looks different.


From Brownells- When cutting a chamfer, you normally do not want to have the internal di-
ameter of the rear-most portion of the chamfered area to be over .020" larger
than the diameter of the bullet of the cartridge that is being used. In other
words, in a .38 Special revolver, which utilizes a bullet that is normally .356"
to .358" in diameter, the maximum diameter of the chamfer should be
.376" to .378".
Just something to compare to the other guns.

[justindad]

Am sure you did but gotta ask... did you range rod the new barrel and cylinder?
What does that mean?

[Three44s]

What does that mean?

A range rod checks cylinder throat to barrel alignment.

Three44s

[Bird]

willy,
if the barrel is rifled to 0.430'', and it slugs at that, then you will need cast bullets that are 0.431'' at a minimum, or 0.432'' diameter bullets.
If you are leading the barrel for the first inch or so, your bullet diameters are too small.. You will need to see what is the largest will fit in your chambers. If the bullet is too small for the chambers, you will also get leading in front of the forcing cone.
Compare the diameters of your cast 180's to your 250's. The bullet bases are most important, and even 0.001'' can make a big difference.

[Bird]

willy,
sorry, did not see that your bullets are 0.431''
See if you can get 0.432'' to fit and try that.
Are you getting leading beyond the cone?

[Larry Gibson]

Keyholes are caused by undersize bullets.

That is only one reason.

The fact that lighter weight and probably shorter cast bullets did not keyhole is evidence the barrel twist is too slow for the longer 250 SWC bullet to fully stabilize at the velocity used.

[TurnipEaterDown]

Several causes to keyholes as mentioned.
Another is poorly sized bullets w/ geometric axis that gets "re-positioned" during sizing. It doesn't take much. I have done personal testing, and in my 44 Rem Mag SBH, if I could see the bullets had a high side to the sizing mark, they (260 LBT WFN, over 20.5 WC820) would be unstable enough to produce several sideways impacts from a full cylinder at 50 yd.