I have just recently made the switch to .40 Limited, and I have been advised by many to load my .40's to 1.20 or even slightly longer. Limiting factors include magazine, and chamber. I have read everything I can find on the subject, but no one really explains why. I understand it is to make the gun function better, by making the rounds more similar in length to 45ACP. But I don't understand the interaction between the load and the longer COL. My bullets end up not seated in the brass as far as I'm comfortable with either. This is a 2011 platform, using 155gr Montana Gold hollowpoints, and VV N320 powder. I am aiming for a power factor just under major for a starting load. VV does not list a bullet even close to this on their charts, and N320 is not covered in the load books I have. But my question is more about the why's of loading long.

I can't be sure without hearing exactly what you were told, but I suspect you are getting bits and pieces of different suggestions that may not exactly go together with YOUR gun and selected bullet.
What you need is a coherent loading that becomes an integral part of an efficient shooting system. I'll try to explain.

First, Bullet Weight:
Although it is possible to make major with 155gr in .40S&W, most USPSA competitors I know, including me, use 180gr. I use Precision Delta Jacketed TC over a near-max load of HP38. Remember. power factor is bullet weight times speed, and it is easier to use a heavier bullet than load to MACH 2.

Second, Loading Data: You will find a lot of proven load data specifically for competition on the Brian Enos forums. There is no need to reinvent the wheel.

Third, Seating depth: Some specific barrels, such as the competition models from STI and SVI, have longer barrel throats and open chambers for better reliability when running fast, hot, and dirty. Many shooters feel it is better to load the bullets longer than SAAMI standard to offset the longer throats and get better accuracy and feed reliability. At any rate, cartridges loaded long for these competition barrels will surely cause jams in barrels with SAAMI standard dimensions.

As the seating depth of a bullet is changed, adjustments are needed to keep the velocity at required Power Factor. In general, the longer out you seat, the lower the pressure and velocity you get.

Finally, Recoil Spring: The recoil spring needs to be matched to the competition load you select. Too strong a spring makes the gun sluggish and can cause feed failures. Too light a spring makes the gun very quick and snappy but batters the life out of it. I saw one race gun slide separate just behind the port and eject the back half of the slide into the shooters lower teeth. Keep a spare recoil spring in your range bag. (I like ISMI for the 1911 platforms).

Once your load is selected and your gun tuned for it, load up a few thousand and try to wear out the gun in the next few years.
Good shooting!

When I regularly shot IPSC Limited class, I shot a Watkins/STI gun in .40, using 220 gr. bullets, loaded quite long. With the 220s, you can load them long enough for good reliability, and the powder space left in the case is about the same as with 175-180 gr. bullets loaded to the standard OAL. BTW, heavy bullets loaded to make major recoil less than light ones, due to requiring less powder to get there. Those 220s at about 800-820 fps. are soft-shooting indeed, and that made Major when Major meant Major With today's power factor requirements, you could run them at 750 and still have a good cushion. FWIW, I ran a 15 lb. recoil spring with my old loads.

Infinity has recommended the bullet weight & COL for a gun I'm having built. And it is being built to run this combination. I'm now running an STI Eagle 5 inch and would like to experiment a bit so the learning curve won't be so steep. I have gotten most of my info from the Brian Enos forum, but I wanted to understand the idea and not just do it because everyone else is. Why does the power go down as the COL goes longer? How deep does a bullet need to be seated to be safe? I understand how to measure what I need to work in the gun, and for now I do not need to make major in a match. But, I want to have a baseline load that I can work up from.

The pressure generated by smokeless powder depends on the size of the space it starts in, and the resistance applied to the expansion of the gases.
Put 5 gr of HP38 in a bowl and light it, and it just burns quickly and produces little pressure. Put that same powder charge in a .40 S&W cartridge behind a 180gr bullet, and you get over 30,000PSI peak.

What Affects What

Among the things that affect the internal performance (pressure and velocity) of a given cartridge & bullet are:

Capacity of the powder chamber (a function of chamber size and shape and case construction)
Relative burning rate and burning characteristics of the propellant powder used
Amount of propellant used and how much it fills the powder chamber
Diameter, weight, and the bearing length of the projectile
Length and interior dimensions of the barrel
Uniformity and speed of ignition of the propellant powder (Primer and loading density related)
Temperature of the propellant prior to ignition
Barrel freebore
Neck tension

Internal Ballistics

Interesting. Do the SVI folks know that you're setting up for IPSC, and not IDPA? Most of the folks I know who use 155 gr. bullets in .40 are IDPA shooters, loading to a PF of only 125-130. If you try to load 155s out very far, you're going to run into problems with insufficient neck tension, since you'll have so little bearing surface inside the case, and that is an issue, whether you're shooting Minor or Major.

SVI is likely recommending the longer OAL for reliability in their mags, which are .45 ACP/10mm/.38 Super in depth, front-to-back, with no spacer for .40s. I could be wrong, but I think that 155gr. bullets are going to be too short to load for much increase in OAL and still have enough purchase on the bullet. I could easily see bullets getting cocked to one side on feeding and really tying things up. Besides, not every such recommendation is the best; Bill Wilson recommended 22 lb. recoil springs in .40s for years, and most of us just ignored him. (A heavy recoil spring in a .40 is a good idea ONLY if you've got a very light mainspring. Otherwise, 15-17 lbs. is correct.)

BTW, I'd look closely at OKFC's posts, above; he knows whereof he speaks.

I just finished reading the page, that was a lot to digest. They are aware of it's intended use, and have told me their factory limited guns are set up this way. It is to be a 6 inch barrel, and they mentioned the need to get the slide moving. Anyway, I think the article told me what I need to know. I always wondered why the load books were different between .40 and 10MM with the same bullet. And my intent is to start out low and increase, chronying every step of the way. Their recommendation was 4.7-4.8gr of VV N320, loaded to 1.21 COL, and a 155-165gr Montana Gold bullet. They did not specify the profile. My plan is to start around 4.6gr or slightly less, with a 155gr hollowpoint, with a much more conservative overall length because I am using a different gun and do not know whether the barrel will allow it. I already know this is probably a minor PF load.

I built a Limited gun for my best friend's brother, using STI frame and slide, with the full-length and full-profile dustcover on the frame. The slide is heavy, too, but it is only a 5" gun. When he shoots in matches where he knows there will be no chrono, he runs powder puff loads with 155gr. Montana Gold TC bullets, and uses Viht N320 powder. I think he's getting all of 750 fps.! That particular gun has an 11 lb. recoil spring with one coil cut off, so it's about 10-10.5 lbs. The mainspring is a 17 lb. When I was test firing it before he picked it up, it was running 100% with standard-length ammo, and I think he loads to barely over standard OAL. Mighty soft-shooting, but you could smoke a cigarette while the slide is cycling.

I use a SVI 6" and load both the 155 and 175 LSWCs' to 1.165". I don't think I could push the 155s' out much longer. While I only need another 100fps for the 155 to make major I went back to the 175s'. They just feel better to me in my gun. I always went with the understanding that the 1911/2011 platform was designed for the longer rounds and would function better with them. That said I've shot plenty of standard 1.125" in my gun with no problems. Your mileage may vary.

Good advice above.

I've been loading .40 for USPSA competition about a decade now; first an S&W, then a Glock, Para P-16 and now a long-slide (6") SVI.

1. 155 is the wrong weight. Use at LEAST a 170; I'm using 180 grain now and so is almost everyone I know.

2. ANY true 1911 design (read: .45-length mags) will have a magwell longer than .40 cartridges. Ergo, loading long will almost certainly be beneficial. I run 1.200 - 1.250" on my .40 cartridges.

3. HP 38 may work; I've never used it. I run WST under my poly-coated lead bullets and VV N350 under the jacketed ones. Many .40 shooters are quite fond of TiteGroup and I used to use Red Dot. I'd start w/WST; it's versatile, available, clean and meters well.

Start low, work up and shoot safe.