Does the Unnatural Strength 1 gained from synthmuscles stack with a space marine's natural Unnatural Strength 4?

From my shallow research (emphasis on shallow [external link to the 40k Wiki]), Space Marines' Unnatural Strength originates from A) a second heart, B) thickened muscle tissue, and C) more oxygen/nutrient-efficient blood. Synthmuscle would be localized to modifying B, so the question comes down to whether purely artificial muscle (Synthmuscle) can provide more benefit than artifically-assisted-yet-arguably-still-natural muscle (Biscopea muscle). Personally, my answer is "yes", but others may have more insight for you. I am not a fan of Space Marines.

No; it's a rating system, not a bonus system.

That is, it's not "Unnatural Strength +1" but "Unnatural Strength 1". If he's getting a better strength score from something else, he's going to use that instead of the lesser bonus.

Unless it says it stacks - which I do not believe it does in this case.

No; it's a rating system, not a bonus system.

That is, it's not "Unnatural Strength +1" but "Unnatural Strength 1". If he's getting a better strength score from something else, he's going to use that instead of the lesser bonus.

Unless it says it stacks - which I do not believe it does in this case.

This was my thinking, do you have anything to back this up that I can show my player?

From a background perspective, either approach could be argued. Asymptomatic made a good argument here: if synthmuscles provide their bonus by replacing weaker natural muscles with stronger artificial ones, would you really gain something if you replace stronger (SB +4) natural muscles with strong (+1) artificial ones? Or would you perhaps even sacrifice Strength instead? On the other hand, perhaps they do not replace but are added on top of the natural muscles? If so, they might certainly improve even augmented Strength even further.

Ruleswise, I'd say Bore nailed it. If it says +1, it's additive. If there is no +, then it replaces any previous Unnatural Strength with this new value, as a character cannot have two different instances of Unnatural Strength.

its a extra unnatural strength its not that weird, its not like they haven´t written some stuff in the core book wrong before. Is there anywhere else in the books where it says unnatural that don´t stack with your earlier unnatural?

As plausible as it might sound in this case to differentiate between US 1 and US +1 it is by no means consistent and might be proven wrong at another occasion. It is no intended rule and as such is not reliable. The RBs even have rules that are somewhat dubious at times.

But for your Math:

A Marine gains US +4. So US (Synth) 4 + US 4 (Marine) still perfectly results into 8.

Also there are Psypowers that grant you Unnatural Abilites without even mentioning this precious "+" yet clearly claim to improve a body beyond its limits.

There are Profiles for Creatures that cant even decide whether to use +X in parentheses or simply a number for the traits. (P. 365; Grey Knight - Unnatural Strength (4) | P. 369; Flayed One - Unnatural Strength (+4); etc.)

DO NOT interpret something into the way it is written for they simply fail at doing it.

As for this case though I would say yes, they do stack. Why?

Well normal Synthmuscles apply to a human. And a human has already muscles. They are woven into these muscles.

As for a space marine they also do have muscles that fit their body frame, just as a human. They might be big, dense and super but yet they might still benefit from other materials woven into them, why should they not? Astartes can be improved by artifical implants(Bionic Arm much?), so why stop here? There are servitors with vat grown muscles around that easily surpass those of an astartes.

The cool thing about astartes in fluff is not that they are super warriors. The cool thing is that they are super warriors that can be mass produced. Once you managed to get all the implants right you just need to feed them some flakes and milk (Wood will do too) and their body will more or less turn into its perfect shape. You may train a bit but that is more for coordination. Astartes do not lift weights in front of mirrors - at last most of them. Their bodies will be great but nothing that cant be improved further - if you manage to get your hands on that resources. In terms of strength they are nothing special - great but not the pinnacle. Ask some TechPriest. He might have no trouble at all to craft you a Servitor on Roids that lifts more than an astartes. Now ask some dark mechanicus adept if he wants to mess with your physiology and enhance it even further. That guy will giggle like a school girl that got asked for prom by the quarterback.

Well normal Synthmuscles apply to a human. And a human has already muscles. They are woven into these muscles.

As for a space marine they also do have muscles that fit their body frame, just as a human. They might be big, dense and super but yet they might still benefit from other materials woven into them, why should they not?

"Diminishing returns", so to speak. For better or worse, Unnatural Strength does not translate perfectly into additional Strength, but merely augments and thus abstracts the end result.

Usually I argue that the game would be better off without Unnaturals and should simply add to the normal characteristics, but in this case it could be interpreted as going against an individual's natural limitations. Space Marines are already augmented, so it gets harder to augment them further. In other words: "Unnatural Strength 1" might mean less to them than the difference it could make to a normal Human, the efficiency of this upgrade possibly halved, rounded down, and disappearing from the equation. The improved version (which also adds a Test penalty) that adds US4 might be afflicted in the same way, but at least add a +2 to the existing numbers.

But that is really just gut-feeling based on my interpretation that the end result of muscle augmentations partially depends on what was already there.

Either way, you have a point when it comes to the flawed presentation of the stats, so I would have to retract my statement regarding the book being clear about it. Upon re-reading this implant's description, the book also says that synthmuscles are "woven into" and thus added to organic muscles rather than replacing them entirely, so it could well be meant as a stacking bonus.

A Marine gains US +4. So US (Synth) 4 + US 4 (Marine) still perfectly results into 8.

What do you mean by "still"? If the bonuses do stack, the Strength Bonus end result (assuming the improved version of synthmuscles that also impose a -10 Test modifier) would be SB 12, no? (4 natural Strength, +4 augmented Strength, +4 synthmuscles)

I would be inclined to agree with Lynata. You can go either way, it's not super clear in rules as written. As far as the logic is concerned, you could argue that you're just sewing into some additional muscle, and anyone even a Space Marine can benefit from having some extra super muscles.

But personally after all the effort and resources that goes into selecting, training, and modifying a Space Marine. If they could could all be bumped another strength bonus by a comparatively widely available surgery. Wouldn't that be a standard augmentation? That idea is hardly iron clad, because there's a number of cheap useful implants they don't have by default. But all the same when I read it I always took it as "This is something mortals do to get stronger like a space marine. To match something they really can't match naturally." The +4 SB variation especially. It's what comes to mind when in the novels or the game books they describe humans having slabs of vat grown muscle that makes them look weird. Rather than a sort of universally applicable surgery.

The fact that they're not available in Death Watch's core rule book would seem to suggest that at some point at least one person agreed that this was their intent. All the same if you want to make that available, then go for it. "They are extra large variations made special for space marine types like you!" Maybe bump up the rarity or cost if it makes you feel better. Personally I'd just say no, keep the implant as something of a band aid for mortals to try and catch up.