45-70 velocities with FFFg and 405 grain bullets

[Erasmus]

I have a new Winchester 1886 and decided to put together some black powder .45-70 loads for it. I didn't have enough FFg on hand to make more than a few rounds, so used FFFg instead.

I loaded some with new Starline brass with CCI LRP, a Chevy Cast 405 grain black powder lubed bullet, and either 4.0CC (60 grains) or 4.3CC (65ish grains) of Goex FFFg, with or without a soda box cardboard wad. (i.e. some 4.0CC charges had wads, some didn't; ditto for 4.3CC). I used a compression die to compress the powder column such that the bullet would compress it the last 1/10th of an inch or so. The rounds drop into the chamber fine (plunk test) and I didn't see any rifling marks on them when I extracted a live round.

I took the photo gate chronograph out (an older Chrony) and shot over it and saw some, to my eyes, crazy velocities. I started seeing 1700 fps with 60 grain loads. I was expecting closer to 1300 fps and was taken aback.

Could it be my chronograph was acting up? Maybe BP smoke was causing a discrepancy? It seemed to be working well enough when I chronographed some .45 ACP earlier in the day.

Could I have screwed up my compression die setting and compressed the powder column too far and created an air space under the bullet?

Anyway, I have enough FFg at the moment to load a few rounds with that, but then my buddy shot my chronograph that day so...

[Kent Shootwell]

Those numbers do seem a bit off, try this calculator for comparison.
https://www.p-max.uk/cgi-bin/black_powder.cgi
How close was your start gate to the muzzle?

[Erasmus]

I was a paced 10 feet from the muzzle. I thought about pushing back further but just never did for whatever reason.

[Erasmus]

That calculator is really great.

Running through the calculator with Swiss FFFg and (not measured but looked up) case capacities of 77.5 grains of water it figured on about 1150 fps at 17,000 psi.

I'm just not seeing any way I could have hit 1700 fps with FFFg in a .45-70. Going by that calculator, even if I'd managed to put 85 grains by weight FFFg I shouldn't be breaking 1400 fps with the 24 inch barrel. Even 85 grains of FFFFg wouldn't break 1500 fps. I'm nearly convinced I just had an issue with the chronograph and black powder smoke.

[Lucky R. K.]

I have had the card wad distort readings when testing black powder loads. Unburned grains of powder can also affect the readings. I use an old Chrony.
Lucky 

[rust]

Try a paper sheet on the first screen.

[Erasmus]

If only someone had mentioned that to me earlier. First, I might have gotten better readings; and secondly, I (or my buddy) might not have shot the back of my chrono!

[Kent Shootwell]

I set my sky screens up at 15 feet for black powder any thing. The velocity is still close enough for my needs but what I'm interested in is how consistent they are. That distance with cartridges using wads or muzzle loaders using patch's works well.