Was thumbing through my Speer No 11 this morning and noticed that for the 257 Roberts they have different data for their 100gr flat based and boattail bullets. In some cases, the boattail data shows a full grain greater charge and over 100fps less velocity. I don't recall seeing this discrepancy in any other load manual and it doesn't show up in any other manual that I have on hand. Seems counter intuitive since the boattail should have less bore bearing surface. Any ideas?

Makes sense if you figure a boat tail of the same weight probably has less bearing surface and thus less pressure

The Speer softpoint boattails are swaged, and feature a lead alloy with less antimony (and hence softer) the flat-base Hot-Cors. My bet is they "obturate" (expand more upon firing) than the harder-core Hot-Cors.

I recall Speer also advise different load data for the 100 grain SP and the 100 grain Grand Slam in their older manuals.. and also for the 105 gr SP and the 105 grain RN...these being 6mm bullets

Less bearing surface is exactly why you should expect lower velocity from the same load.

Because powders burn progressively, the more resistance a bullet meets going down the bore, the more pressure builds behind the bullet accelerating burn and boosting pressure.

The Speers are more accurate in a few of my older rifles than bullets by other manufacturers. I have always believed that they do obturate fully, unlike some others. I am almost certain that is due to the softer cores.

So I ran into another head scratcher in the same manual. Loaded up a bunch of plinking rounds in 38spl. 3.6gr of 700X behind 148grWC.This is the max in this manual. Decided to put some 357Mag together with the same components. The max load is 3.0gr of 700X They used a 6 inch Ruger for both loads, possibly the same revolver. Can't understand why a case with larger volume would take a lesser max charge. If they did use the same revolver I could understand if the bullet traveled farther before reaching the forcing cone in a 357mag cylinder it would produce less pressure but then what happens when you shoot that load in a 38 cylinder? BTW I've shot hundreds of rounds of that 38 load in both 38 and 357 revolvers with no problems.

If those are lead bullet loads, the Speer manual artificially limits those loads to low velocity to avoid leading; most of them are not even close to the pressure limits.

You can successfully push cast bullets significantly faster than the Speer manual data if the bullets are properly sized and the right alloy & hardness, but Speer plays on the safe side with that data, I think to accomodate use of softer swaged bullets.

That, and occasionally one runs into load data which makes absolutely no sense and is obviously a misprint.

I know not the explanation for the 38 sp vs 357 data previously mentioned.

Yes it baffles me and the 357 load shows higher velocity with less of the same powder.

Fire off an email and ask. They will get back to you.

https://www.speer-ammo.com/support/contact-us

The 38 has more jump to the forcing cone which acts like free bore in a rifle. The 38 also has a slightly longer time of pressure bleed off in a revolver due to the slightly longer time of travel. But see what the technicians say. This is conjecture but light loads in the larger case will have a larger area for the primer to ignite, more surface contact between the powder and flame which can increase initial peak pressure.

The manuals tell you the results they got that day with the components and firearms they used.
They're not going to tell you anything other than that.

The other problem with some older Speer (and some other manufacturers' data) is they did the load work-up in a pressure barrel, with pretty closely controlled SAAMI chamber and bore dimensions. Then they chronographed the pressure-developed loads in a factory rifle, which in at least one manual Speer claimed was done in order to show "typical" sporter velocities.

The problem with this is mass-produced rifles tend to have varying chamber/bore dimensions, which of course results in wider velocity variations. In one Speer manual this "technique" resulted in higher velocities from the .308 Winchester than the .30-06, the reason they added an explanation for the anomaly.

Luckily, most manual data these days, both pressure and velocity, comes from the same pressure barrel.