Having done some testing of this scenario, I would have to agree with the sentiment of the OP regarding the 45 mm guns. All AT guns (not just the 45 mm) seems to be very good at landing shells near the infantry, even when both they and the gun are on a relatively flat field. In reality such proficiency would be very difficult to achieve as AT guns are very low to the ground and fire high velocity shells along a relatively flat trajectory. These attributes are great for hitting targets which possess a large profile which rises considerably above the terrain level (any vehicle, bunkers, buildings etc), but are a huge impediment when trying to engage infantry, except in very specific circumstances (targets on forward hill slope, for instance). This is because targeting infantry with HE weapons essentially requires that the gunner lob his shells so they hit the ground near to the infantry, thus even the slightest error in range information will result in the shell overshooting or undershooting the target by a large distance. I've noticed this effect myself while playing SABOW; even with a cannon that has a much larger blast radius and is mounted a lot higher above the ground it often takes a fair bit of trial and error to land a shell on a cluster of infantry. I usually find it far easier to eliminate exposed inf with MG fire as you can just walk your tracers onto the target and chew them to pieces in a matter of seconds
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I also agree that the suppression produced by the 45 mm is too strong, with just a single shell needing to land within the general area of an inf squad to pin it down completely for a lengthy period of time after it hits; IMHO sporadic hits should not pin them down for this long, only fairly intense sustained fire. This combined with the 45 mm's high ROF and the aforementioned AI uber gunnery allows an unsupported 45 mm gun to devastate an entire squad/s with HE in short order even if they're moving quickly and in a dispersed single line formation, as even infantry with high morale stop moving and returning fire after the first shell or 2 lands in their vicinity (even if they're not that close and produce no casualties), and simply lie there getting pounded until the squad morale breaks and they panic.
From what I've read the 45 mm when used in an infantry support role was primarily used to destroy fixed fighting positions such as bunkers, although its main role was for destroying lightly armoured vehicles. It certainly wasn't the weapon of choice for pinning down/destroying attacking infantry, machine guns and mortars were far more effective in this role. Currently in GTOS it significantly outclasses either of these weapons in their intended role, and allows players to not bother protecting their guns with inf as is done in real life as they can pretty much fend for themselves.