Although pressure is certainly related to velocity, there are various factors that make it less than a 1 to 1 relationship. Bullet friction related to type (jacketed, plated, lead), OAL, primer effectiveness, powder burn speed, case capacity variances and case neck tension, etc. will all have effects and may cause more or less pressure relative to velocity.
I believe JSG said less than a 1 to 1 relationship. Now, where to begin? I agree and especially with fast burning powders where the relationship between pressure and velocity are more likely to be near the 1 to 1 ratio. Move down to slower burning powders with higher energy potential and things change for the better in terms of the velocity you can expect at a given pressure level like say the 9mm's Max. pressure rating per SAAMI at 35,000 PSI/33,000 CUP. For a very fast burner, Bullseye actually has pretty good energy potential. The problem is that once you hit its Max. potential or slightly exceed it, pressures can spike.
noylj, brings up a good point in his .44 Magnum reference. When you increase the powder charge and don't get any gain in velocity, this is referred to as the "point of diminishing returns." Pressure may be spiking or very close to doing so at this point. As you move to slower burning powders with good pressure stability, the less likely that is to occur and why the IPSC guys use a very select few spherical powders to load 9mm MAJOR. HS-6, True Blue, Silhouette, 3N37 and more lately, AutoComp. Others see some use like LongShot and while it was available in the US, Vectan SP-2 was used quite a lot. With any of these powders you can build fast and safe high velocity 9mm. What you CAN'T do is load 9mm MAJOR for typical 9mm pistols which takes down another street: OACL. Most IPSC competitors use custom built 1911s with long chamber throats where the magazines will allow longer OACL's than the SAAMI Max. spec of 1.169". Some common 9mm pistols have long throats (i.e. S/S P-226/229/228 and G19 & 17s) but OACL is restricted by the magazine. Unfortunately, there are 9mm pistols with short barrel throats like the CZs and XD/XDm, so it is imperative that you learn how to establish the proper OACL for a particular bullet in the pistol it's to be fired from.
So, 1Victor30, Nick has already given a pretty good explanation of things. You're getting higher than load manual velocities for a couple of reasons. Alliant tested with a 4" barrel where you're using a 5" and in my experience, XD/XDms tend to give high velocities in relation to their barrel length. I've always wondered where the Croats are getting their steel, if it's actually produced in Croatia and probably is, it makes for some pretty darn good barrel steel. Getting 1278 FPS with a 5" barrel from a load rated 1180 FPS from a 4" test barrel is still quite an increase. You guys had the chrono about 12' from the muzzle didn't you?
Let's get to a couple of issues that have been raised: lowering pressure by increasing OACL and bearing surface. Typically, load manuals list short OACL recommendations and handloads should never be shorter, IMO. This is stated in a number of manuals and by powder makers who give data for their powders. It's kind of a worse case scenario where the data providers are intentionally using the shortest OACL practical to establish Max. pressure ratings. What JSG was referring to in regards to bearing surface is that different bullets have shanks of differing lengths, and the longer the shank is, the more bearing surface it has with the barrels rifling. As noylj pointed out, velocity in regard to bearing surface can change also and I'll give an example. In my experience, the Montana Gold 124 gr. JHP is about .0005" or 1/2 thousandth gives me higher velocity with a given powder charge than most any other bullet. So with its slightly smaller diameter, bearing surface is decreased as is the coefficient of friction in the bore. For this reason, whenever I work up 9mm +P loads, I tend to use SIERRA's data because their 125 gr. JHP has about the longest shank you'll encounter among the various 124/125 gr. JHPs, and further, SIERRA loads them very short.
Now, I have a couple of questions for you, Victor. Have you established that the 1.150" OACL with the 115 gr. FMJ is safe in the 5" XD's chamber? And, do you really need as much velocity using an FMJ bullet? Again, as Nick pointed out, you can answer the proper OACL question by doing a "plunk" test. A handload of slightly shorter OACL than the Max. allowable will give an audible "plunk" when you drop the handload into the chamber, with the barrel removed and muzzle tilted downward. But you can also establish the Maxx. Allowable OACL for a given bullet in a given chamber fairly easily. I generally find that by using a jacketed bullet, you can just barely start that bullet into a FIRED case. Take this "dummy" and insert it into the chamber and then push on the case-rim until the case stops forward movement. This is where the case-mouth has contacted the forward ledge in the chamber and the barrel's throat is seating the bullet. Make up about 5 of these "dummies" and test them all in this manner. You should get a consistent Max. Allowable OACL. Then, when you make your handloads, shorten them by .005 - .010" below the Max. allowable you established with the "dummies." Part of this will also depend on the OACL variation that your press and dies allow. Typically, I just shorten to the next lower
.5mm depending on which or whose pistol I'm loading for, so here's how that shakes out: 1.102"/28mm, 1.122"/28.5mm, 1.142"/29mm and finally, my longest loads are 1.161"/29.5mm which I like to use for 147 gr. JHPs. But again, you have to establish that none of these lengths are too short for YOUR pistol's chamber. For CZs and XD/XDms using my favorite 124 gr. Rem. JHP (not Golden Saber), 1.122"/28.5mm works and is just a hair longer than many factory JHP loads.
The only bullets I load in 9mm currently are JHPs. The 124 gr. Montana Gold does not expand in my testing, so it's little different than using an FMJ. It may be possible that Montana Gold's FMJ bullets use the same nominal diameter I'm finding at around .3545". If so, they'll also produce higher velocity with a given charge of powder as will bullets with shorter shanks. Like I said, bullets like the 125 gr. SIERRA JHP and some other truncated cone shape bullets like the XTP have fairly long shanks in comparison to JHPs that have a rounded ogive. Here's one example of a handload I use: the 124 gr. Rem. JHP in once fired nickel +P cases with a CCI-500 and the +P charge of Silhouette I use with an OACL of 1.142" chronograpks 1238 FPS from the 4.14" barrel in my Rugr SR9.
Now we get to the confusion. Around 1990, give or take, SAAMI reduced the Max. pressure rating of the 9 x 19mm from 35,700 CUP to 35,000 PSI/33,000 CUP while establishing the Max. for 9mm +P at 38,500 PSI. Since loading the 9mm since the mid 80s, I've pretty well concluded through careful study of data and observation of pressure signs that there is little difference between 9mm +P at 38,500 PSI and older data where the former Max. of 35,700 CUP was used. My problem with the SAAMI reduction is in since we know the established Max. for +P, why is there so very little data for it? In fact, I haven't seen anyone give 9mm +P data in over 10 years and the last to do it was Ramshot. So for you guys that may not have seen the older data, here's a FYI specifically in the case of HS-6. Today's standard Max. Standard Pressure loads are a good deal below the older loads when the pressure rating was 35,700 CUP. I won't give a specific charge, but take my word for it that the charges with HS-6 were a good deal higher than anything you'll see today. In the SPEER #11, they used a Beretta 92SB and the data is from the mid 80s. The Max. Charge of HS-6 gave them a velocity of 1315 FPS and supposedly with either their 115 gr. JHP or FMJ and by today's standard's would easily be rated +P and not too far away from what some have rated +P+ using 115 gr. JHPs at 1350 FPS like the Illinois State Police, ISP, or Secret Service loads of yesterday. The ISP used the Win. 115 gr. JHP and the SS load used the 115 gr. Rem.
In summary, whenever your loading close to or at Max. Charges, a chronograph is very useful. And going back to the general discussion, whatever powder you're using, when you observe NO velocity gain with an increase in powder charge, you've hit PEAK pressure or maybe just beyond it and described as the "point of diminishing returns." I don't recommend using anything faster burning than say Unique for higher velocity loads and my personal preference is for spherical powders that have better presure stability. For both the 9 x 19mm as well as the .40 S&W, what I recommend for full power loads are the sphericals starting at V-V N330 down to AA#7 or LongShot depending on whose burn rate chart you're looking at and how they have AA#7 and LongShot placed. They reverse order in different charts. In regard to my own +P type loads, I've never had any issues following the recommendations for powders used in IPSC/USPSA 9mm MAJOR. I don't load anywhere near those levels and the only OACL restriction I have with the SR9 is the magazine. They have long throats and that's why I prefer them personally. I'm not recommending that any of you attempt 9 x 19mm +P handloads because the loading parameters are a good bit narrower and because of the lack of data. I just don't have any need to push an FMJ, or a cast lead bullet for that matter, beyond the velocity that standard pressure loads are capable of. JHPs are a different matter entirely and I do handload my own defense loads at +P levels, at least in terms of velocity. I know that +P is a pressure designation, but sometimes it is used as a marketing tool. In the case of the Win. Ranger 127 gr. +P+, it would require a less than optimal powder selection for it to be +P+ in terms of actual pressure. While there is no SAAMI recommendation for Max. pressure for 9mm +P+, there is the suggestion that it shouldn't exceed 40,000 PSI and I'd really be surprised if the 127 gr. +P+ Ranger is that close to 40,000 PSI. Another example is the SPEER 124 gr. +P Gold Dot. SPEER rates it at 1220 FPS and from my SR9 that load chrono's 1174 FPS. I have a handload with the same Rem. 124 gr. JHP mentioned previously with a charge of Silhouette just higher than SIERRA's recommendation where I'm using a longer OACL of 1.122"/28.5mm where their's is 1.075" and my handload gives the exact same velocity over the chrono at 1174 FPS and because of the longer OACL, I don't believe my handload would actually pressure test above 35,000 PSI for standard presure 9 x 19mm. Of course, to make that definitive, it would have to be pressure tested. The charge I use for that load is
.5 grs. lighter than the charge I use in my defense load that chrono's 1238 FPS. Again, there are good and bad powder selections to accomplish such a load. HS-6 is plenty capable but tends to flash higher than the others. Power Pistol is capable but I don't use flake powders and in particular, double-based flake powders like Power Pistol or Blue Dot that will give impressive muzzle-flash if you like that sort of thing. True Blue comes by low flash kind of naturally through it's high volumetric density and very small physical size. 3N37 works very well with low flash and what I used in my defense loads prior to moving to Silhouette with the Rem. 124 gr. JHP. For higher velocity 147 gr. JHP loads, few are better than AA#7 that was designed for such purpose, i.e. high velocity submachinegun loads with heavier 9mm bullets and V-V 3N38 can also work for that.