I usually shoot FMJ/CMJ bullets and have recently picked up some polymer coated bullets to try out. I had to drop the powder charges down a bit to get the same PF. However, at the same PF, I noticed that the POI of the polymer coated bullets were lower than the jacketd bullets. Is this normal?

 

I have fixed sights on my M&P and it seems that I have to get a shorter front sight if I want to keep using the polymer bullets with the same POA/POI that I'm used to from jacketed bullets.

Are both types of bullets the same weight?

 

Are these those cast lead bullets that are "painted" with polymer?

Yes and Ye

Are both types of bullets the same weight?

 

Are these those cast lead bullets that are "painted" with polymer?

 

Yes and Yes.

All bullets will shoot slightly differently. Even two different manufacturers jacketed bullets at the same weight will shoot slightly differently. You can adjust your sights or adjust your load or adjust your point of aim.

Sounds like they give different pressures in the barrel. Maybe go by lead data.

No doubt the lead obturates better but if they're the same weight and velocity, it shouldn't matter what the pressures are. If he's dialed in separate loads that will give both bullets the same velocity at the same point downrange (wherever the chronograph is) I don't understand how one would drop more than the other...

 

How far apart are the two groups?

The design of the bullet will matter. The profile of the nose, the profile of the gas and how gases affect it as it leaves the barrel, all these things matter. No two bullets designs will fly the exact same except by blind luck.

No doubt the lead obturates better but if they're the same weight and velocity, it shouldn't matter what the pressures are. If he's dialed in separate loads that will give both bullets the same velocity at the same point downrange (wherever the chronograph is) I don't understand how one would drop more than the other...

 

How far apart are the two groups?

 

I re-read the OP and realized he adjusted the charge. but Vlad is right, a possibility is more drag from one bullet to the other since they use different outer materials, that is IF they are exactly the same bullet shape/weight.

All bullets will shoot slightly differently. Even two different manufacturers jacketed bullets at the same weight will shoot slightly differently. You can adjust your sights or adjust your load or adjust your point of aim.

 

Generally speaking, how would load affect POI? Hotter load = higher POI?

No doubt the lead obturates better but if they're the same weight and velocity, it shouldn't matter what the pressures are. If he's dialed in separate loads that will give both bullets the same velocity at the same point downrange (wherever the chronograph is) I don't understand how one would drop more than the other...

 

How far apart are the two groups?

 

The polymer bullets were about 1-2" low @ 20yards. My POA is at the top of the front sight, I had to change my POA to the FO to get similar POI. I tried the polymer bullets in two different guns/calibers and have similar results. 180gr out of my M&P40 Pro w/ KKM barrel and 124gr out of my 5" 1911 Spartan 9mm.

 

I just ordered a short front sight for my M&P and I my Spartan has adjustable rears, so I should be ok. I just wanted to know if my results were typical when comparing FMJ to Lead/Moly/Poly bullets.

 

Also, I went from 5.0gr N320 (170pf) to 4.35gr N320 (174 pf) when changing from Zero FMJ to the polymer bullets. 4.0gr N320 was right around 166pf. I'll probably end up around 4.2gr to target 170pf.

Generally speaking, how would load affect POI? Hotter load = higher POI?

 

Bulklets leave a gun going uphill (the barrel is tilted upwards relative to the plane of the sights) so that the arc of rising and falling intersects the sight picture. Depending on range, without adusting the sights, you get different points of impact relative to the point of aim. Speed a bullet up and you stretch the arc and thus shift the POI relative to the point of aim of the sight picture which was unchanged between the faster and slower bullets.

 

If the bullet has effectively different drag, you change the shape of the arc. In theory this should effectively be rounding error and not matter with handgun projectiles, but it does, and some are different enough that you notice.