I'm about 90% sure this is a dumb question, but I feel the need to ask it anyway.

I've decided I'm going to start traveling a bit to some different USPSA events.  I've signed up for the Western States Single Stack Championship in Arizona.  I've only ever shot competitively around Wyoming and Montana, at 5000 - 5500' ASL, and Costa Mesa is at 98' ASL.  My load is right on the low end of what qualifies for Major Power Factor.  So the question is, will the denser atmosphere of the lower elevation cause me to lose velocity in any measurable way?  It would be a pain in the ass to spend two days driving to a match and then not make power factor.

yes.  chrono it there or use a good ballistic calculator that has altitude inputs to give you the differences.

I am no expert, but altitude was one thing I remembered from my last class that does matter.

Air is thinner at altitude, this reduces drag on the bullet at longer ranges. Less drift and drop compared to sea level.

Small (1500 - 2000) foot changes won't have anywhere near the effect that a 6,000 to 9,000 foot change will.

The opposite is also true. If you live and develop your loads at 6k or 7k feet don't be surprised when you're at sea level when you need more elevation and windage for identical conditions.

Very much so, I worked up my load at sea level, choreographed it, had it down.  Went to a class at 5300 ft. I had to completely disregard my previous ballistics and get new 'come ups'.

It's not going to have much effect on a pistol bullet chronographed a few feet from the muzzle. If you're shooting a load on the cheesy end of major, add a couple tenths to the charge and it should be good. You're probably fine as things are. For precision rifle that change would be significant in terms of elevation correction.

Thank you.

I agree that it would be a major difference with a precision rifle, but we're talking about 45acp.  Maybe I should have mentioned that.

I think it may have been a lack of familiarity with USPSA and power factor among respondents. And yes, big difference between flying through denser air for a few feet and doing it for a few hundred yards.

I've heard several stories of people in CA. going to matches in AZ. and not making Major and getting thrown into Open. I've never heard a definite reason, but my guess would be its a different chronograph.

The Field Density Altitude Compensator (FDAC) says YES.

Learning about density altitude will explain all.

At the muzzle, no.

Down range, yes.  The air is less dense at altitude.

air density changes velocity

so... yes, altitude changes velocity

for a pistol at close ranges though, it won't matter much

for distance work with a rifle, it would be a dramatic difference

Yes...I shoot at home at sea level but hunt from 2500feet up over 7000 feet and end up running everything over the crony and verifying both zero and dope before hunting for each area...

It could have been for any number of reasons, and all of them potentially connected to trying to just barely make numbers.

It will have a major effect on intermediate ballistics at long range. Not much effect on what you might measure as velocity at or near the muzzle, but a significant effect on the flight path of your bullet.

Nope, not much difference at pistol ranges.

Gotta be something else going on there.  I'm a USPSA Range Officer.  By the rules, if you don't make Major Power Factor, you'll just be assigned to Minor, which is scored differently.  If you fail to make Minor, you can usually still shoot, but can't receive a score.  Single Stack is a special case, because it's all 1911's and they must fit in a specific box to be legal.  Minor allows you 10 rounds in the magazines, Major only allows 8.  A 10 round 45 magazine won't fit in the box, but the 10 round 9mm will.  So if you're shooting 45acp with Minor Power Factor, you've got several disadvantages to overcome.

I did have a friend who got tossed in to Open at the Area 1 shoot.  He was shooting Production, which limits magazine capacity to 10, and he fired an 11th and 12th shot.  That was a tough day for him.

There's some great responses to this thread.  Sadly, I can't take the time to go down there and test my ammo before the match.  I have to make a minimum of 830fps, so I think I'll load them to where I'm getting about 875 - 880 up here and it should be just fine down there.  Unless someone comes along and tells me that's not enough, that is.

The main factor that affects mv given the load and gun stay the same is temperature. Some powders are more sensor than others, some not at all. A change in altitude or da should not change mv any significant amount.

At distance shooting at different density altitudes very much affect ballistics. It's because of changes in how velocity bleeds off though, not because of changes in mv.

Yeah I probably got something mixed up but they were penalized

Yes it matters and yes you can be moved into minor for not making PF.

Density altitude will effect even pistols, the guys telling you it won’t aren’t running ammo right on the border of velocity minimums. It’s not only DA but the ambient temp having an effect on your powders burn rate that can effect your rounds. The trick is to have a load that maintains requirements for Major PF across the board of where you shoot/compete. Don’t run it right on the border of legal, bump it up a bit.

Another option is changing your load based on location/predicted conditions but that doesn’t seem like the juice is worth the squeeze.

Dusting off physics neurons that haven't been used in 35 years, different altitudes will have a negligible effect on muzzle velocity.

In the case of 45ACP, you have 21,000 PSI at the most on one side of the bullet and anywhere from 26 to 29 psi on the other side. 3 pounds of difference is only 0.014%. 9mm would be .009% and 223 Rem .005%.  That would be very hard, if not impossible, to see on any consumer crony in 3 shots.

With that said, air pressure after the bullet leaves the muzzle has a much larger impact on the bullet's deceleration, so the higher the real ambient pressure the quicker the bullet loses velocity.  The effect is more pronounced the faster the bullet is traveling.  Rifle bullets at 3000fps will slow down much faster than pistol bullets at 850fps.

With power factor being measured 5 feet from the muzzle, I'd be really surprised if you could detect any variation without 100s of shots being fired.  Your standard deviation in velocities will most likely swamp any differences in deceleration caused by more or less air pressure at that range.

The difference in rifle bullet drop at 1000 yards might be measured in feet at different density altitudes and at that range, air pressure is a big deal.  Retained energy and current bullet velocity will also take a big hit.  The trajectories will be completely different past a few hundred yards.

But my understanding of your question was pistol power factor for an USPSA match and I don't think you could tell the difference at different altitudes.  ***edit:  But if it makes you feel better add 0.1gr to your load.

Higher altitude has less dense air, lower elevation has more dense air, meaning it slows the bullet faster at low elevations then at high elevations...

The margin you would use to stay out of trouble in general will be enough for .45 and 9mm to go to sea level.

The DA has an important effect for sure, but you are measuring at the muzzle, so temp will play a bigger role.

Just make sure you keep enough margin for conditions in general and it will cover you for traveling to sea level.

Good Luck at your match.

Nobody is contesting that. However, context dictates that it's rather less of an issue when determining muzzle velocity of a pistol.

This.

I agree with you....but if consistency is the name of the game in competition, isn't it something worth at least measuring and knowing what the difference is? Not a competitor, but if I was, I think I would want to know, just so I know...