and .998 caliber for heavy bolters
This comes from a misunderstanding of the top right image of Codex : Space Marines from 4th Edition, on page 11.
It was labelled "998. model bolter: Godwyn [...]" (and it's explicitely a .75 calibre).
Unfortunately, a mistake was made - I think it was during the Dawn of War video games era? - by adding a decimal point in front of it when describing the Boltgun weapons... and it became .998 model [...].
As it is a known fact that the latter pattern of Astartes Boltguns were .75 calibre, it was conceived as an error and was attributed to the calibre of the Heavy Bolter instead.
But we have no lore about the "real" calibre of an Astartes Heavy Bolter (unless there is some genuine official lore, somewhere, that states that Heavy Bolters are .X calibre that I am not aware of. If that's the case, please do point me to it, I'd be happy to be proven wrong!). It was only described, multiple times, as being "Enormous versions of the boltgun, heavy bolters are deadly large weapons that fire fist-sized bolts at the enemy." (c.f. Codex : Dark Angels, 4th Edition, p49. Codex : Blood Angels, 5th Edition, page 57, and probably in other places).
Another example would be the Land Speeder picture on page 27 of the 4th Edition Dark Angels Codex. It is described as having a "997AV Godwyn pattern pintle-mounted Heavy Bolter" but it is a "model/designation" number, not a calibre.
regular bolters are .75 caliber for both humans and space marines
There are multiple calibres for Boltguns. From .50 (e.g. Ikanos Pattern Bolt Pistol) to .75 (Godwyn Pattern Boltgun). The Tigrus Pattern Boltgun is .60 calibre, and the Phobos Pattern Boltgun is .70 calibre, for example.
(The Horus Heresy Book 1 - Betrayal).
And "regular" humans do not commonly use the .75 calibre Boltguns like the Astartes. Even the Sisters of Silence can not operate such weapons and use a much lower caliber.
"[...]. More gunfire sounded as Torunda’s Sisters attacked the enemy from the rear. Two of the Word Bearers turned about to face the new foe. The bolters the null maidens carried were of a much smaller calibre than those of the Space Marines, and struggled to penetrate Astartes battleplate, but one of the giants went down under concentrated fire from the squad, only wounded, but out of the fight. [...]" - (Throne of Light, Chapter Eight).
And we're talking about the Null-Maidens here...
"[...]. The Sisters of Silence are utterly fanatical in their cause. This is the product of a regime of harsh martial discipline, indoctrination and chemical therapies, besides which even the methods of the Legiones Astartes pale in comparison – all so that they may endure the horrors they must witness in their task." - The Horus Heresy Book VII - Inferno)
"Regular" humans use a smaller calibre of Bolt weapons (in a smaller (i.e. human) sized weapon).
"[...]. ‘What do you think it is?’ Yassilli asked. She had fetched her own weapon, a bolt pistol that she had to carry with both hands, though it was small calibre, designed for standard humans to use. ‘It’s beautiful,’ she said, watching the cold fires dance. [...] - (Throne of Light, Chapter Thirty-Six).
"[...]. The Space Marines are not the only warriors of the Imperium to carry boltguns into battle, but the version carried by the Adeptus Astartes, the MK Vb Godwyn pattern boltgun, is by far the largest and most devastating. By comparison, the smaller patterns of boltgun carried by the Adeptus Sororitas or the champions of the Imperial Guard are pale reflections. So large is the Godwyn pattern that no normal man could heft one, let alone survive its unforgiving recoil. [...]" - (Munitorum - Volume 1)