bullet weight and velocity vs poi

[webrx]

Hi again,

Another question, in trying to work up some 45 colt loads for my 1858s, I notice with 250fr lrn and 5gr of trailboss, everything shoots 4-5" high at 25 yards.

My question is, all other things being equal, for a 250gr lrn how does velocity affect poi if at all i.e. 650 fps vs 850 fps will loading for a slower or faster round shoot to a lower poi?

Similar question for bullet weight, say a 200gr vs a 250gr loaded to the same velocity, will one shoot to a lower poi than the other?

Do I need to load a combination of slow velocity and light bullets to get the poi closer to poa?

I am trying to work up the most accurate load for these revolvers and any advice is appreciated.

Thanks,

Dave

 

[TimSr]

For a pistol, the faster the bullet, the lower the point of impact. Bullet weight is not really a factor, other than heavier bullets are typically loaded slower than light bullets.

 

[GooseGestapo]

Actually, weight (actually, mass to be correct) does matter.
With a given weight, it's the factored muzzle rise and barrel travel duration that determines the muzzle angle when the bullet exits that determines how the impact point is affected.
At 850fps, a 270gr will hit higher than a 255gr at same velocity.
A 200gr at same velocity will hit much lower.

With a rifle and it's much greater mass, the opposite effect occurs. Due to generally higher velocities, the lighter bullets drop less at shorter range, therefore impacting higher.

For the stated reasons, I prefer my .45's to run the 255's at about 875, and 270's at about 950.
This gives me consistent poi at 25-50yds. I typically use the Lee 255gr RFN, and RCBS 270gr SAA.

 

[LeftyTSGC]

I use 5.0 of TB in my .45lc (Uberti 1873SAA) I get a good POI from mine. I do adjust my sight picture off the front blade to compensate for the slight bullet climb but mine is manageable. But you can try to increase the load, maybe 5.5gn of TB to increase the velocity. I believe the Max is at 5.8, but with TB there is not a lot of danger.

LeftyTSGC

 

[CraigC]

The rule of thumb is:

For a given velocity, heavier bullets will impact higher.

For a given bullet weight, lower velocities will impact higher.

 

[webrx]

Thanks,

If I am understanding correctly, from the info provided, the determining factor is basically how long the bullet stays in the barrel and the pressure it takes to drive the bullet (which causes more muzzle flip?).

So, assuming both bullets are moving at 750fps, the heavier bullet takes more pressure to move at that speed than the lighter bullet, more pressure causes more muzzle flip, which makes the bullet impact higher.

For bullets of the same weight, slower velocity keeps the bullet in the barrel longer, which causes more muzzle flip. This one confuses me a little, as if I speed up that heavy bullet, wouldn't that cause higher pressure, which in turn would cause more muzzle flip? Do these offset each other?

So, does a change (decrease) bullet weight (say 250 to 200) do more to lower impact than an increase in speed (say 750 to 850) assuming a 250gr bullet?

I thin I am going to test this with 4.5gr and 5.5gr of Trailboss (should be a ~100fps speed change) and see if it changes poi for the same 250gr lrn,

If I can find some lead 200gr loads I will try those as well.

Thanks again,

Dave

 

[BigBore45]

go with the 200 grain bullet. the POI is close to a 140 grain round ball at 800-900 FPS. i would start at 5.5 grains of trailboss with a 200 grain bullet

 

[CraigC]

I wouldn't strain my brain worrying about "why". It's not a hard & fast rule and there are many factors involved. IMHO, the differences are all less obvious and sometimes even non-existent at lower recoil levels.

Bullet weight usually has a greater effect than small differences in velocity.

 

[webrx]

Thanks again.

d