Need advise from those experienced in fire lapping out a S&W bbl constriction

Ian

Notorious member
Vastly different pressure curves, Bill. Most people think BE burns like TG actually burns, but in reality, BE burns more like Longshot at shotgun pressures.

Three things to consider: Peak pressure value, where that peak occurs, and the slope trailing from peak to muzzle exit. Consider the following three predictions, all having approximately the same muzzle velocity:

44Mag40TG.png

9356PSI max, at .400" of bullet travel, 1692PSI muzzle.

44Mag41BE.png
Here we see BE peaking pressure at 7842PSI and .430" of bullet travel, and muzzle pressure up at 2101PSI, indicating a much longer burn. Also note BE did not completely burn in this scenario, whereas TG was 100% burned up at 3", or halfway through the barrel.

44Mag59LS.png

Longshot has virtually the same internal ballistics as BE in this scenario, only peak pressure is slightly lower and muzzle pressure slightly higher, and less was burned. Pmax occurred at .440".

When you think about keeping peak pressure low to minimize base bumping and wanting pressure behind the lapping slug to stay UP toward the muzzle to help keep the seal, you can see why the pressure curves suggest you stick with BE, or Longshot, and use Titegroup for all of your suppressed .45 ACP loads.
 

Pistolero

Well-Known Member
OK, I figured you'd be able to explain that more completely. I was looking at muzzle
velocities and powder quantities only. Very interesting, especially when you are doing
something odd like this where MV isn't the only item of interest, as it usually is.

Looks like I need to get Quickload.

This is an 8 3/8" bbl. Will be using 240 gr swaged bullets. If I seat them normally (far more
comfortable with that than pushing down onto the powder) what charge would you recommend?

Just for grins, what does QL think the pressure would do with say, 4.1 gr BE and bullet at normal
LOA, vs pushed down to contact with powder as one source recommended? Seems like a big
potential pressure increase, but with an unsized case, maybe not so much. Don't plan on doing
it, anyway.

I have buckets of Clays, too, but IME it is faster than TG. Might it be suitable?

Bill
 
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Ian

Notorious member
Here is seated flush, still only 68% full of powder:

44Mag41BEflush.png

And here's down on it:

44Mag41BE100.png
 
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JSH

Active Member
I lapped a Ruger Bisley 45 Colt SS for a fellow a few years back. I think we ran almost thirty rounds through it. We were shooting at a target at about 20 yards, just patterns no group. Then it happened, they all started coming together! I wanted to quit and he wanted to keep going,lol. So we split the difference and ran three more through with finer compound, no change. That is where it was left.
I wish I had a time lapsed video of that outing.
 

Pistolero

Well-Known Member
Yeah, seated flush runs the pressure from max 7800 psi up to 19500 psi, and pushing it down
onto the powder punches that up to 33400, which in a .44 Spl gun would be problematic,
not serious in the .44 Mag. This is exactly the trend I expected, good to see some numbers
on it. I will be seating at normal LOA.

Thanks. Quickload is pretty useful. Going to be on my shopping list.

Interesting, Jeff. Sounds like we need to set up a target and shoot groups while
we fire lap, as the link, and your experience said.

Bill
 
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Brad

Benevolent Overlord and site owner
Staff member
QL is amazing. Being able to see wat happens when you change seating depth or try a different powder is quite helpful.
 

Ian

Notorious member
I have heard the argument for seating flush (albeit with less powder than my examples here) being that the pressure peaks while the bullet is still mostly in the throat instead of in the forcing cone, but as you can see the pressure is still higher at the forcing cone than even max pressure if seated normally using the same powder charge.

I should point out that the predictions are for a closed system, not a revolver with cylinder gap, leaky cylinder throat, and two forcing cones for the bullet to traverse in the first inch. In reality, the initial pressure will be less as the bullet has little resistance exiting the cylinder (other than its mass resisting acceleration) and then spike as the bullet squeezes into the bore.
 

Brad

Benevolent Overlord and site owner
Staff member
And with no neck tension, unsized case, the pressure curve is changed even more.
Having no tension is a good part of why the deep seating is a good thing, help the pressure rise more "normally".
 
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Pistolero

Well-Known Member
Anyone think that 10 BHN is too hard for lapping? I have a some Hornady swaged, certainly
soft enough, but only around 25 of them. If I have to go past that, the softest that I have
already cast up is 10 BHN about 40 of those on hand.

Checked the bbl of my "{new" 1923 vintage .44 Spl, which shoots very well with all that I
have tried so far. Zero thread choke. They used to know (and care) how to fit those
barrels correctly.

Bill
 

Pistolero

Well-Known Member
Tested a number of different loads which had worked well in my guns.

Six different loads, including one with Lee 310gr GC, best group about 2.3" at 25 yds,
worst at about 7", the GC load was about 3". This is after recutting forcing cone and
recutting the muzzle crown.

Now I have some cases fired in that gun, will load up some lapping rounds with 320 grit
to start.

Pin gauges show that a .416 pin will drop freely until it hits the thread restriction, a .414
will pass through the restriction. So, about 0.0015 or so restriction. Cyl throats are all
at 0.432. The test loads were sized to .4315.

This one was particularly odd.... this is an LBT 260, and clearly one of them was really
yawing horribly. No leading in the gun with any of these loads, too. Barrel looks beautiful,
smooth inside.

LBT260_10uniq.jpg

Bill
 
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Pistolero

Well-Known Member
This is the best group we got. Normally this will shoot into 1" or a touch over for me in
several guns.

Keith250_10Uniq.jpg
 

Brad

Benevolent Overlord and site owner
Staff member
The yaw is a sure sign the constriction sized the bullet enough to be out of engagement with the rifling. Under spun or out of balance at that point.

I bet it shoots wonderful once lapped.
 

Pistolero

Well-Known Member
She looks like she wants to shoot but just can't quite get it done. that one LBT bullet
looks like it wouldn't go much farther before hitting dead sideways.

It will be interesting.

The owner also has a Python that isn't accurate. No apparent thread restriction on that
one, tho. :headscratch:

Bill
 
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fiver

Well-Known Member
the deep seating won't affect the BE so much.
it will however sky rocket pressures with the titegroup.